Filename Extensions List

So you see a file somewhere on the 'net with a name like "greatfile.xyz" and you would like to download it and use it on your computer. Will it work? What does the file extension xyz mean, anyway? This handy guide attempts to provide answers to those questions.

It is not meant to be read straight through; rather, it is meant to be a reference. An extension may be looked up either with the "find in page" option of your browser or by appending a "#xyz" (without the quotes and with xyz replaced by the extension in question) to the "go to" or "URL" field on your browser. There is also a Search Interface that will return not only the specific extension sought but also other entries that reference it. Be aware though that it assumes familiarity with the computer basics discussed on the terms page.

You may notice that most extensions are three letters (or fewer) long. This is due to a historical limitation of the operating system called CP/M (that was later inherited by MS-DOS). In fact, the whole concept of file extensions comes from CP/M. Most modern operating systems do not attribute any special meaning to the "." (period, or dot) character.

Be aware though that there is no standardization to filename extension usage, and many different people have used extensions to apply to many different things. This list only attempts to provide likely guesses of what something is apt to be. Programs that can make use of many of these extensions can be found on the Guide to Free Software.

If you want something added or see a problem with something already here (but keep in mind this guide is not meant to be overly technical) please send e-mail to the webmaster.

3dt

A Firebird database file holding a 3D representation of information usable by an organizational software application called Topicscape. It is ASCII and can only be used on machines with both Firebird and Topicscape.

abw

An AbiWord word processing document; it can be read on any machine with AbiWord installed (available for WinTel boxes, Mac OS X, BeOS, and most UNIXes for free).

adf

The Amiga disk file stores the contents of an entire Amiga floppy in a single file. It is binary, but is designed to work on any machine that has Amiga emulator software (Amiga emulators are available for many platforms, including UNIX, Macs, and WinTel boxes) as well as (of course) the Amiga itself.

afm

The Adobe Font Metric file is used to provide information on Adobe fonts. It is in simple ASCII.

ai

An Adobe Illustrator document. It is binary but will work directly on any machine that has Adobe Illustrator.

aiff & aif

The audio interchange file format originated on Macs but has since been ported to several other platforms as well.

apd

An Aldus PageMaker document. It is binary but will work directly on any machine that has Aldus PageMaker.

arc & ark

A file that has been compressed with the arc program or similar will get the "arc" (and very rarely the "ark") extension. Its performance is somewhat less than gzip & zip, but decompression programs exist on virtually every platform. Like zip, arc does not require a separate tarring step; it uses its own method to do the equivalent.

arj

Similar in all respects to arc, but a different method of compression was used. Programs to uncompress this type of file do not exist on as many platforms as they do for the original arc.

art

An image format (typically called the Johnson-Grace format) with extremely aggressive compression at the expense of quality. This format is most frequently seen by AOL users as AOL automatically compresses online images of other formats (like gif or jpeg) into Johnson-Grace images. This is why AOL users often do not see web pages at the same quality level as other people, often seeing blurry images where others see clear images (and occasionally even seeing black bars that are not really present in images).

asp

A proprietary solution that performs more or less like shtml.

au

An audio format that originated on UNIX systems but has since been ported to virtually every other platform out there from the Newton on up. Note though that these files can be either stereo or mono; stereo is not supported on as many machines as mono.

avi

A video file will typically store a movie clip. It's generally supported by many different platforms, although note that there are several different versions of AVI files in use, and not all players will play all versions.

awk

Probably an AWK script; it can be made to run on any machine that has awk.

bar

The bar format is similar in concept to the tar format, but is not found on as many computer systems.

bat

A file with this extension is most likely a batch file for either CP/M or MS-DOS and is not likely to work on a different platform than the one for which it was originally written. It should be simple ASCII however, so it might be possible to figure out what it is supposed to do and write a new script to do the same.

bdf

The Bitmap Distribution Format is a format for storing bitmapped fonts. It is in simple ASCII. It originated on the X-Windows system but can be made to work on other systems as well. It is a product of Adobe.

bep

A file that has been compressed with geoBEAP. It can be used on GEOS that have geoBEAP installed.

bmp

Microsoft bitmap image format. This is a fairly simple bitmap format that can be viewed on many different platforms but is directly supported by very few browsers. It is not used too much on the web (fortunately, as it is very inefficient) with GIFs, JPEGs, PNGs, and even X-bitmaps being preferred as they are much more portable. It is somewhat similar to the pict format.

bsh

Probably either a Bourne Shell script or a Bourne-Again Shell script. These are both pretty similar to each other, anyway (Bourne-Again is a superset of Bourne), and the Bourne-Again Shell is available for many different platforms.

bz2

The bzip2 compression program is similar in nature to both gzip and zip. It will often outperform the other two, but it is not yet particularly widespread and so is not usually the best choice for information that must be shared between different platforms.

c

A source file written in the C programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a C compiler. There is a good chance though that it will also require (at minimum) some h files.

C, cpp, & cxx

A source file written in the C++ programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a C++ compiler. There is a good chance though that it will also require (at minimum) some H (or even h) files.

cgi

A common gateway interface file is a program designed to be run over the web. It is really something else in disguise, like a Perl script, an AWK script, a Bourne Shell script, a Java program, or whatever.

cgm & metafile

A computer graphics metafile is an image file. Programs for handling metafiles can be obtained for most platforms.

chr

This is a rather generic sort of extension indicating "character" data; of particular note though is the fact that Borland has a WinTel-only font format that uses this extension.

com

A command file is a program for a CP/M, MS-DOS, MS-Windows, or possibly even an x86 GEOS machine. It will probably not work on a platform other than the one for which it was designed (including the different variants of MS-Windows). Similar to, but usually smaller than, an exe file.

cpt

A file that has been compressed with the Compact Pro program will get the "cpt" extension. A less likely possibility is a special type of Zope Page Template called a Controller Page Template that is used for generating online forms.

crt

There are two types of unrelated files that use this suffix. The more common type is designed to hold certificates used to authenticate resources sent over the Internet; the less common type is a UNIX crontab file used to schedule automatic repeating tasks.

csh

Probably a C-Shell script. This will probably be difficult to make work on non-UNIX systems, but it will at least be in simple ASCII so it might be possible to figure out what it is doing and rewrite it.

css

A cascading style sheet file is used to define a cascading style sheet for a web page. The purpose is to provide more control over the fonts, colors, layout, etc. that go into the web page than could be provided by raw HTML. Also, since the cascading style sheet file is separate from the HTML files, it can be shared (or even inherited; a little outside the scope of this document) by multiple web pages to help provide a consistent look-and-feel across a web site. It is not yet fully supported by all browsers; newer versions of all popular browsers do provide some CSS support, however.

cvt

GEOS files have external resources associated with them; files with this extension are typically packaged from a GEOS environment for transmission.

cwk

A ClarisWorks (or AppleWorks) document (can be a word processing, spreadsheet, database, presentation program, drawing program, etc. document); can be read on any machine with ClarisWorks installed (available only for WinTel boxes and Macs, but usually not for free).

cwl

A ClarisWorks (or AppleWorks) library can be read on any machine with ClarisWorks installed (available only for WinTel boxes and Macs, but usually not for free). There are some minor differences though that will sometimes cause a library written with a particular version of ClarisWorks (or AppleWorks) to be read-only if used with a different version.

cws

Pretty much like a cwk file, but indicates stationery rather than an ordinary document.

d00

This file is designed to work on any machine that has C64 emulator software (C64 emulators are available for many platforms, including UNIX, Macs, and WinTel boxes) as well as (of course) the C64 (or C128) itself. It is essentially a repackaging of a regular Commodore delta file.

d64

The C64 disk file stores the contents of an entire C64 floppy in a single file. It is binary, but is designed to work on any machine that has C64 emulator software (C64 emulators are available for many platforms, including UNIX, Macs, and WinTel boxes) as well as (of course) the C64 (or C128) itself.

del

A delta file used (rarely) by C64 or C128 computers for data storage. Delta files are identical to the much more common sequential data file, differing only in name. Sometimes these are referred to as "deleted" files, but this is not a particularly good term as delta files can perform active duty and have nothing to do with deletion.

dir

Some systems use the "dir" extension for directories or folders.

dmf

The DMF format is used for storing vector fonts. It originated on Amiga systems and will not work on most other machines without conversion.

dmg

A dmg file is a Mac OS disk image that can be mounted as if it were a hardware disk.

doc

This is a rather generic sort of extension indicating some sort of document, usually in simple ASCII; of particular note though is the fact that both old versions of FrameMaker and MS-Word often save their native format files with this extension, and such a file may only be read with FrameMaker or FrameViewer (if a Frame document) or MS-Word (if an MS-Word document). Neither of these programs are available for all platforms, and even when they are available are usually not free. To complicate matters even further, there are several different versions of MS-Word in common use that cannot reliably read each others' files. Thus in general this format should not be viewed as portable, and chances of reading it on any particular platform are slim. Note that newer versions of FrameMaker save documents with the FM extension and Microsoft is splitting its MS-Word format into two variants that use either the DOCX or the DOCM extension. Documents saved from either FrameMaker or MS-Word that are meant for other machines should probably be saved in MIF or RTF formats (respectively) in any case.

docm

This extension is used by one of Microsoft's replacements for the DOC format. It can only be read by machines with MS-Word (at least version 12) installed. Note that unlike the DOCX variant, these files can contain embedded code (the "m" stands for "macro") and should thus not be considered safe if they come from untrusted sources.

docx

This extension is used by one of Microsoft's replacements for the DOC format. It can only be read by machines with MS-Word (at least version 12) installed. It is a safer sibling to the DOCM format.

dtml

A DTML file with dynamic extensions that provide for dynamic capabilities. DTML files are often used with Python files in implementing dynamic web sites.

dwf

The DEC Windows Format is a format for storing bitmapped fonts that originated on DEC UNIX systems and will not work on most other machines without conversion. It is a product of the company PageStream.

egg

Python Egg files are similar to JAR files but for Python rather than Java.

el

An Emacs LISP file is used to extend the capabilities of the popular Emacs editor. It is ASCII and will work on any machine with Emacs installed.

elc

A Emacs LISP file (compiled) is used to extend the capabilities of the popular Emacs editor. It is a binary, compiled version of a .el file.

emf

An MS-Windows enhanced metafile is like an ordinary CGM metafile but with extensions that make it specific to MS-Windows, and it will not work on most other machines without conversion.

eps

An encapsulated Postscript file is often a combination of Postscript and various miscellaneous image file formats and is often difficult to use on a set up that is different from the one on which it was created.

etd

An Adobe e-book download file is not an e-book itself but rather a series of instructions that a computer will be able to use to download an e-book and load it into an e-book reader. It is binary and not particularly portable.

exe

An executable is a program for a CP/M, MS-DOS, MS-Windows, or possibly even an x86 GEOS machine. It will probably not work on a platform other than the one for which it was designed (including the different variants of MS-Windows). Similar to, but usually larger than, a com file.

f3 & f3b

The Sun Folio format is used for storing vector fonts. It originated on Sun UNIX systems but will work on many X-Windows systems.

f66, f77, & f90

A source file written in the ForTran programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a ForTran compiler. The only caution is that the number refers to the version of ForTran used.

faq

A FAQ file is traditionally a simple ASCII document that attempts to answer frequently asked questions.

fes

A format exported by an organizational software application called Topicscape. It is in simple ASCII and can thus be viewed on pretty much any machine, although it can only be usefully imported into Topicscape.

fla

A Macromedia Flash source document. Can be used with any machine that has the commercial Macromedia Flash editor, currently available for only Macintosh and MS-Windows.

fm

This indicates a FrameMaker native format document. Such a file may only be read with FrameMaker or FrameViewer or similar (currently only available for a price on UNIX, Mac, & WinTel platforms).

fnc

This file is a Frogans Network Certificate. It is a small file used to permit the initialization of a Frogans network.

fpx

The FlashPix image format was developed by Kodak but is now governed by an independent organization called the Digital Imaging Group. Programs for viewing this format are available for a few different platforms. An extended version of this format can also handle audio, but this variant is not too widely supported at this time.

ftk

The IBM Triton FastTrack format is an audio format. It is not too widely recognized.

gam

A TADS (Text Adventure Development System) data file. Typically it will represent an interactive fiction story (or interactive tutorial, or similar). It is binary but will work on any machine with some flavor of TADS interpreter, and such interpreters are available for several different platforms, usually for free. It is currently not as portable as a Z-machine file.

gdf

A GEOS dictionary file. It is (obviously) meant to be used with GEOS.

ged

This file is used for holding genealogy data, and is most typically called a GedCom file (short for genealogy data communications). Most software applications made for performing genealogy work (or even just basic family trees) are capable of working with GedCom files and they can generally be used on all platforms. Note however that some companies have made proprietary extensions to the GedCom format, so transfers from one software application to another are not completely guaranteed to be safe.

geo

A GeoWorks document; it can be used on any machine with GeoWorks installed (which in turn requires GEOS).

gf

The Generic Font format is used to store font information. It will work on any machine that has TEX installed.

gif

A general image format file is a representation of an image or simple animation. The format was created by Compuserve and is copyrighted. It is currently the most widely accepted image format in existence, but the concern over its copyright has inspired many people to try and move away from its use. Some popular alternatives include JPEGs, PNGs, and X-bitmaps for still images and MPEGs and QuickTime movies for animations. A GIF can only contain two-hundred fifty-six different colors, of which "transparent" is an allowed color. Each GIF can use its own set of 256 colors, though.

graffle

An OmniGraffle file is a type of XML format. It is used to hold charts and diagrams.

gz & z

The GNU Zip program is used to compress the size of a single file. If more than one file has to be compressed, they must be tarred together first. The program that does the compression is called gzip, and the gzipped file cannot be used until it has been gunzipped. The gzip program has its origins on UNIX machines, but it has been ported to most other computer types as well and is typically available for free under the terms of the GNU public license. The "gz" form is preferred; the other is considered obsolete as it was frequently confused with the output of the UNIX compress program and deliberately changed to be something more distinctive. Gzip is better at compression than compress.

H, hpp, & hxx

A header file written in the C++ programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a C++ compiler. Typically it would be used in conjunction with a C file. Note that the h extension is also sometimes used for C++ header files.

h

A header file written in either the C, C++, Objective-C, Objective-C++, or Inform programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a C (or C++, ObjC, or ObjC++ as appropriate) compiler. Typically it would be used in conjunction with a c or C file.

hdf

The hierarchical data format supports the storage of many different data types, including images and tables. Tools for using HDFs are available for most higher-end machines, including most UNIX systems, Macs with a PowerPC processor, and Windows NT & Windows '95 systems that possess at least a Pentium processor. There is also a Java interface for handling HDFs.

hex

This is a generic sort of extension indicating some sort of hexadecimal (or even binary) data; of particular note though is that interactive fiction data files for use with the Hugo program typically use this extension.

hqx

This indicates a BinHex file. It is similar in theory to a uuencoded file, but was developed originally for the Macintosh. Today decoders can be found for MS-Windows, MS-DOS, and UNIX in addition to the Mac.

htc

An HTML Component file is a proprietary way of encapsulating a particular behavior to be attached to a web page. It can always be replaced with standard HTML, CSS, and JavaScript.

html & htm

A hypertext markup language file contains hypertext capable of being read and interpreted by a browser. The bulk of the world wide web is in HTML format.

ics

A file with this extension normally contains calendar information. It should be in simple ASCII and should be usable on any machine with software that supports the iCalendar standard.

iff

The image file format was originally created for the Amiga but can now be read by numerous different image manipulation programs for several different platforms.

inf

A source file written in the Inform language (a language optimized for creating interactive fiction). Inform compilers have been ported to virtually every platform in use today, and will create either Z-machine code or Glulx code depending upon the specifics of the compiler.

jar

A Java archive should be usable on any machine with Java.

java

A file with this extension should be a file written in the Java programming language. It should be in simple ASCII (although also possibly Unicode) and should be usable on any machine with a Java compiler.

jbig & jbg

An image format developed by the Joint Bi-level Image Experts Group. It is optimized for FAX type images, and has absolutely nothing to do with either JPEG or MPEG in spite of the similarity of names. It is lossless and free viewing programs are available for it for most platforms, but it is not directly recognized by most browsers.

jhtml

Very similar to shtml but utilizes Java to do its work.

jpeg & jpg

An image format optimized for "natural" images developed by the Joint Photographic Experts Group, JPEGs are probably second only to GIFs in level of acceptance. JPEGs manage to capture wonderfully detailed images in millions of colors in minimal space by taking advantage of limitations with human vision, and performing little lossy compressions. This means that each time a JPEG is saved, it will lose a little more quality, although each individual loss will be nearly invisible to the human eye. If care is not taken, however, a JPEG image can become worse looking than a color-limited GIF. JPEGs are also not good at storing cartoon-like images or line drawings; for these cases either GIFs or PNGs or X-bitmaps are better choices. JPEGs also cannot contain a transparent color. If a transparent color is needed, GIFs, PNGs, and X-bitmaps are better choices. JPEG has nothing to do with either JBIG or MPEG in spite of the similarity of names.

js

A file with this extension is probably written in the JavaScript programming language for browsers. It should be in simple ASCII and should be usable on any machine with a browser that supports JavaScript. Note that there is no relationship between JavaScript and Java; it is an unfortunate coincidence of names.

key

An Apple Keynote document. It is a compound document built from XML files (and various types of media files) and will work on any machine that has Apple Keynote installed.

koa

Usually a Koala Paint file; an image format originally designed for the Commodore 64, but readable by many other machines as well with the appropriate software.

kon

This is most likely part of a Yahoo! (formerly Konfabulator) Widget. It will be simple ASCII typically containing a mix of XML and JavaScript.

ksh

Probably a Korn Shell script. It should work on most machines that have a working Korn Shell. It can usually also be made to work under the Bourne-Again Shell with only minor modifications.

kth

An Apple Keynote theme file. It is an XML format and will work on any machine that has Apple Keynote installed.

lha

A lharc file is similar in all respects to an arc file, but a different method of compression was used. Programs to uncompress this type of file do not exist on as many platforms as they do for arc.

lib

This is a generic sort of extension indicating "library" data; of particular note though is that native Amiga font files often use this extension.

lit

This indicates a Microsoft Reader e-book. It is currently only known to work with some (although not all) flavors of MS-Windows (including Windows CE), and is binary in nature.

lnx

A LYNX file is similar in all respects to an arc file, but a different method of compression was used. Programs to uncompress this type of file do not exist on as many platforms as they do for arc.

ltf

This file is a Frogans short-cut (or link to Frogans). It is used to indicate a Frogans URL on the Internet. It can only be opened with a Frogans player.

lwf

An image format designed to minimize file size while maintaining image quality. LWFs can utilize both lossy and lossless compressions. Typically they will be better compressed than the equivalent JPEGs if the lossy form is utilized. As of this writing, virtually no browsers have the built-in support to handle LWFs, but separate plug-ins are available for many.

m

A source file written in the Objective-C programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with an Objective-C compiler. There is a good chance though that it will also require (at minimum) some h files.

M & mm

A source file written in the Objective-C++ programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with an Objective-C++ compiler. There is a good chance though that it will also require (at minimum) some H or (h) files.

mdb

A Microsoft database file is a binary file containing a database for use with Microsoft Access. It is not portable and in fact it requires specialty tools to extract the data.

mf

The MetaFont format is used to store font information. It will work on any machine that has TEX installed.

midi & mid

A musical instrument digital interface file is an audio file that will possibly work with any device that is MIDI aware. MIDI is in some respects similar to MOD, but it is supported on far more dedicated musical devices but far fewer computers.

mif

A Maker interchange format file is a Framemaker file with an advertised format so that it can be imported into other desktop publishing or word processing programs. The ability to handle MIFs is built into better word processors and desktop publishers. It will behave the same across different platforms.

miff

An image format used with the ImageMagick suite of image processing programs. It can be used on any platform that supports ImageMagick.

mng

A multiple-image network graphics file is used for storing animations. It is binary and related to the png format.

mod

The audio module file originated on the Amiga and takes the approach of defining different instruments and then describing what each does. It is fairly space-efficient, and free software to play MODs is available for WinTel boxes, Macintoshes, UNIX boxes, and even Commodore 128s in addition to the original Amiga.

mov & qt

A QuickTime movie can actually be used to store both movies and sounds. A file with this extension could thus be either an audio file or a movie file. Both types are supported by many different platforms. Note though that there are several different versions of QuickTime files in use, and not all players will play all versions.

mpeg, mpg, mpeg2, mpeg3, mpeg4, mp2, mp3, mp4, m3u, & m4u

The Moving Pictures Expert Group devised formats for storing both movies and sounds. A file with this extension could thus be either an audio file or a movie file. Both types are supported by many different platforms, with perhaps the audio type being a little more recognized. Both types also utilize lossy compression that is designed to take advantage of the limitations of human vision and hearing. Regardless the compression still occasionally does weird things producing "MPEG compression funnies", particularly during periods of rapid movement in video. MPEGs are not limited to computers; some digital satellite television transmissions use an MPEG format, and DVDs use an MPEG format. MPEG has nothing to do with either JPEG or JBIG in spite of the similarity of names. MPEG does have some relation to QuickTime, though, and the next version of each will work to merge the two. If there is a number at the end of the extension, it refers to the specific generation of the MPEG format in use within the file.

nbk

Probably a Newton backup. It is binary and not particularly portable, as in fact it could have been written by any one of a few different programs (such as Newton Connection Utilities or Newton Backup Utility on either Mac or MS-Windows) which are not guaranteed to work with the files generated by each other.

odb

An OpenDocument database file is the database format portion of the OASIS OpenDocument standard. It is usable by many different database applications.

odc

An OpenDocument chart file is a format used for storing chart data and is part of the OASIS OpenDocument standard. It is usable by many different applications.

odf

An OpenDocument formula file is a format used for storing mathematical data and is part of the OASIS OpenDocument standard. It is usable by many different applications.

odg & otg

An OpenDocument graphics file is the drawing format portion of the OASIS OpenDocument standard. It is usable by many different drawing applications. The version with the "d" is an ordinary document; the version with the "t" is a template for building other documents.

odi

An OpenDocument image file is a format used for storing image data and is part of the OASIS OpenDocument standard. It is usable by many different applications.

odp & otp

An OpenDocument presentation file is the presentation format portion of the OASIS OpenDocument standard. It is usable by many different presentation applications. The version with the "d" is an ordinary document; the version with the "t" is a template for building other documents.

ods & ots

An OpenDocument spreadsheet file is the spreadsheet format portion of the OASIS OpenDocument standard. It is usable by many different spreadsheet applications. The version with the "d" is an ordinary document; the version with the "t" is a template for building other documents.

odt & ott

An OpenDocument text file is the word-processing format portion of the OASIS OpenDocument standard. It is usable by many different word-processing applications. The version with the "d" is an ordinary document; the version with the "t" is a template for building other documents.

ogg

The Ogg Vorbis format is an advanced binary audio format similar to (but more capable than) the mp3 format. It is patent and royalty free, and is supported on most platforms.

opf

An open e-book package will use this extension. It is binary, but applications to read it exist or are being developed for a number of different platforms.

p00

pages

An Apple Pages document. It is a compound document built from XML files (and various types of media files) and will work on any machine that has Apple Pages installed.

pas

A source file written in the Pascal programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a Pascal compiler.

pbm

The portable bitmap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. It is similar to pgm but can only handle monochrome images. There is also a binary variant that is more space efficient but less portable.

pcf

The Portable Compiled Format is a format for storing bitmapped fonts. It is binary, but still designed to be portable. It originated on the X-Windows system but can be made to work on other systems as well.

pdb & prc

Probably a Palm package of some type, either a Palm Database or a Palm Resource. In either case, it is basically a program made to run on the Palm platform, and will sometimes be a Palm DOC type of e-book. It is binary and not particularly portable, but applications exist to read Palm DOCs on a few other platforms.

pdd

The PhotoDeluxe Data image format was developed by Adobe for use with its PhotoDeluxe program and can be read by any machine that has it installed (it is only available for Macs and WinTel boxes, and then generally not for free).

pdf

The Portable Document Format is a format for storing all manner of documents. PDF documents can contain sounds and animations in addition to simple images and are binary in nature. PDFs are also fully capable of containing hypertext, but unlike HTML will guarantee proper printout. It is a product of Adobe and free PDF interpreters are available for most machine types. There are also free PDF plug-ins available for Netscape and compatible browsers.

pfa

The PostScript Font ASCII is a format for storing vector fonts. It is simple ASCII and originated on the X-Windows system. It is a creation of Adobe and the ASCII counterpart to the less widely recognized PFB.

pfb

The PostScript Font Binary is a format for storing vector fonts. It is binary but designed to be portable. It originated on the X-Windows system. It is a creation of Adobe and the binary counterpart to PFA.

pfm

The Printer Font Metric file is used to provide font information on an MS-Windows machine. It probably will not be useful anywhere else.

pgm

The portable graymap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. It is similar to ppm but can only handle shades of gray. There is also a binary variant that is more space efficient but less portable.

php, php3, & php4

Very similar to shtml but utilizes PHP to do its work.

phtml

Very similar to shtml but utilizes Perl to do its work.

pict, pct, & pic

Macintosh bitmap picture format. This is a fairly simple bitmap format that can be viewed on many different platforms but is directly supported by very few browsers. It is not used too much on the web with GIFs, JPEGs, PNGs, and even X-bitmaps being preferred. It is somewhat similar to the bmp format, although utilizes compression and so boasts smaller file sizes.

pif

This extension can be used for more than one thing. First is the process interchange format, a temporary file used to facilitate communication between different programs. There is also a rarely seen picture interchange format which can be used to store images. These two formats are both binary but can be used on pretty much any platform that has the appropriate support. The third possibility is a Microsoft program information file; it can be used only on MS-Windows machines and is in reality just what is more commonly called an alias, link, or shortcut.

pk & pxl

The TEX Packed font format is used for storing bitmapped fonts. It is binary but will work directly on any machine that has TEX. The PXL form is obsolete; all current files should be PK.

pkg

Probably a Newton package of some type; a program made to run on the Newton platform. It will frequently be a Newton book type of e-book or a Newton Script program. It is binary and not particularly portable, but applications have been written to read Newton books on a few other platforms. Note that somewhat unfortunately, this extension is also used for the unrelated Apple package format used on Macs.

pl

This extension will typically mean one of two completely different things. The more common is a Perl script, a program written in the Perl language. It should run on any machine that has Perl installed and is simple ASCII. The second is a TEX Property List file. It is used to provide information on fonts. It is also simple ASCII, so it should not be too hard to tell the two apart.

pm, pm3, pm4, & pm5

An Adobe PageMaker document. It is binary but will work directly on any machine that has Adobe PageMaker (currently only available for Macs and WinTel boxes).

png

The portable network graphics image format is designed to replace the GIF. In fact, PNG is sometimes jokingly said to really stand for "PNG's not GIF". PNG is completely lossless and can handle millions of colors; it is not limited to a palette of two-hundred fifty-six like GIF. It also has full support for transparent colors. Its only real disadvantage is that right now not all browsers support it directly. It has been recommended by the W3C, though, so odds are pretty good that most (if not all) future versions of browsers will provide direct inline support for PNG, and PNG images will start to become more commonplace on the web.

pov

POV-Ray source files will typically be given this extension. POV-Ray source files are simple ASCII and should be readable on most platforms. Effective use can only be made of them on machines with POV-Ray, however.

ppm

The portable pixmap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. There is also a binary variant that is more space efficient but less portable.

ppt

A Microsoft PowerPoint document. It is binary but will work directly on any machine that has Microsoft PowerPoint or Apple Keynote. Note that neither of these products is free, so this is not a good distribution format. Also, Microsoft has split this format into two different variants (one uses the PPTX extension while the other uses the PPTM extension) in newer versions of PowerPoint.

pptm

This extension is used by one of Microsoft's replacements for the PPT format. It can only be read by machines with MS-PowerPoint (at least version 12) installed. Note that unlike the PPTX variant, these files can contain embedded code (the "m" stands for "macro") and should thus not be considered safe if they come from untrusted sources.

pptx

This extension is used by one of Microsoft's replacements for the PPT format. It can only be read by machines with MS-PowerPoint (at least version 12) installed. It is a safer sibling to the PPTM format.

pqa

A Palm query application is used to relate a group of Palm Database files together.

press & pre

Press files are typically source files for the Newton Press program and will work on any computer that has Newton Press (currently available only on Macs and WinTel boxes). Newton Press can save press files as Newton books.

prg

A program file is a program usually for a C64 or C128 computer, although it could also be for something else. It will probably not work on a platform other than the one for which it was designed.

ps

Postscript files are capable of holding quite a variety of information ranging from pictures to text documents to font definitions. Postscript files can be directly interpreted by many printers, and Postscript viewing software comes with many computers and can be obtained for free for most others. The most popular free Postscript viewer is called ghostscript and may be downloaded from many different FTP sites. In truth, Postscript is actually a full language, and Postscript files can perform all manner of things.

psd

The PhotoShop Data image format was developed by Adobe for use with its PhotoShop program and can be read by any machine that has it installed (current versions are only available for Macs and WinTel boxes, and then not for free).

pxr

The Pixar image format was developed by Pixar and can read by numerous different image manipulation programs for several different platforms.

py

Usually a Python script, a program written in the Python language. It should run on any machine that has Python installed and is typically in simple ASCII.

pyc & pyo

Usually a compiled Python script (that is, Python bytecode), a program written in the Python language. It should more or less run on any machine that has Python installed, but unlike py files will probably be binary rather than simple ASCII. In the case of the "pyo" variant, the script is not only compiled but optimized.

pyd

Usually a Python shared library. It is binary and not portable.

qxd

A Quark XPress document. It is binary but will work directly on any machine that has Quark XPress (currently only available for Macs and WinTel boxes).

r00

rar

The rar format is used for archival purposes.

ras

The Sun rasterfile format is an old image file format that is no longer all that common. Many image processing programs are capable of reading this format, however, so it can generally be used on most platforms.

rb

A Rocket e-book file. It is binary and not particularly portable; in fact it is geared primarily toward dedicated e-book hardware.

rel

A relative file used by C64 or C128 computers. Relative files are a complex data storage system directly supported by the C64/128 operating system. It is quite different from the more common sequential data file more typically used by Commodore, and is not supported by any Commodore versions of GEOS.

rle

A Utah run length encoded file image file can be viewed with free viewer programs on most platforms, but will not be directly recognized by most browsers.

rm

A RealMedia movie file will typically store a movie clip. It's generally supported by many different platforms.

rnq

A Renque file contains data representing a discrete event simulation model that can be read by the application Renque (currently available only for MS-Windows). It is a simple ASCII format.

rsrc

A resource file contains data to be used by Mac OS programs and will typically accompany C or C++ source files. It is binary and specific to the Mac.

rst

This is probably a restructured text document. It is like a structured text document in that it is essentially an ordinary text document that uses careful structuring to indicate format, but it uses slightly different rules making the two a little different in the details.

rtf

A rich-text format file attempts to provide more formatting capabilities than simple ASCII while maintaining portability. Unfortunately different versions of RTF are not always 100% compatible, so it is not 100% reliable.

ruby

Usually a Ruby script, a program written in the Ruby language. It should run on any machine that has Ruby installed and is typically in simple ASCII.

s00

scpt

Usually an AppleScript script, a program written in the AppleScript language. It typically runs only on Macs.

sda

A self-dissolving archive contains compressed data that should in theory decompress itself when run. The primary problem is that it will be platform specific and will only decompress properly on the same type of platform that was used to create it. Similar in principle to a self-extracting archive but different in practice. Note too that StarOffice (pre version 6.0) saves its drawing program documents with this extension; these can be read on any machine with StarOffice or OpenOffice installed (available for many platforms including WinTel boxes, Macs, OS/2 boxes, and several types of UNIX boxes).

sdc

A StarOffice (pre version 6.0) spreadsheet document; can be read on any machine with StarOffice or OpenOffice installed (available for many platforms including WinTel boxes, Macs, OS/2 boxes, and several types of UNIX boxes).

sdd

A StarOffice (pre version 6.0) presentation program document; can be read on any machine with StarOffice or OpenOffice installed (available for many platforms including WinTel boxes, Macs, OS/2 boxes, and several types of UNIX boxes).

sdw

A StarOffice (pre version 6.0) word processing document; can be read on any machine with StarOffice or OpenOffice installed (available for many platforms including WinTel boxes, Macs, OS/2 boxes, and several types of UNIX boxes).

sea

A self-extracting archive contains compressed data that should in theory decompress itself when run. The primary problem is that it will be platform specific and will only decompress properly on the same type of platform that was used to create it. Similar in principle to a self-dissolving archive but different in practice.

seb

A Franklin e-book file. It is binary and not particularly portable; in fact it is geared primarily toward dedicated e-book hardware.

seq

This is a generic sort of extension indicating a simple sequential data file. It may be either ASCII or binary, and may contain data from pretty much any character set. A file with this extension may or may not be portable. This particular extension is used heavily on C64 or C128 computers, so there is a high probability that files with it were intended for such a machine.

sfl & sfp

The SoftFont format is used for storing bitmapped fonts. It is a product of Hewlett-Packard and is supported by many HP printers. The SFL variant indicates the font is to be used in landscape orientation. The SFP variant indicates portait orientation.

sfs

The SoftFont Scalable format is used for storing vector fonts. It is a product of Hewlett-Packard and is supported by many HP printers.

sfx

sh

Probably a Bourne Shell script. It will work on any machine that has the Bourne Shell or Bourne-Again Shell installed.

shar & sha

Groups of files are often sharred together so that they may be handled as a single file. A file that ends with this extension is probably such a file and will have to be unsharred into its individual components before use. The whole concept of sharring started on UNIX machines, but it is also possible to get programs to shar and unshar on most other computers.

shtml & shm

An HTML file utilizing server-side includes. An SHTML file can be somewhat dynamic; a typical use is the automatic inclusion of a "last modified" date.

sid

A SID audio file is a program that will directly run on a computer with a SID chip (like a C64 or C128 or a WinTel box with appropriate hardware). As an audio format, it is fairly efficient. Furthermore, free SID players are available for most platforms, and there are even free SID playing plug-ins available for Netscape and compatible browsers.

sit

A file that has been compressed with Aladdin's Stuffit program will get the "sit" extension. It is similar in performance to gzip, but less portable. Unlike gzip, sit does not require a separate tarring step; it uses its own method to do the equivalent.

snf

The Server Normal Format is a format for storing bitmapped fonts. It is machine-dependent; that is, do not expect that a file in this format from one machine will work on another. Virtually every machine that uses SNF fonts will be able to convert them to BDF, though.

so, dll, & dylib

A shared library object (also called a dynamically loaded library or dynamic library). Whatever they are called, these things are binary and extremely platform-specific; they may fail to be usable even on two machines of the same model and OS based upon other less obvious specifics.

spd

The Speedo format is used for storing vector fonts. It originated on the X-Windows system and is a creation of the Bitstream, Inc. company.

stx

This is probably a structured text document. It is like an ordinary text document but utilizes careful structuring to indicate format.

svg

A Scalable Vector Graphics file contains XML data representing an image. It can be viewed by any application that can understand SVG data.

swf

A playable Macromedia Flash animation. Playable on any machine with the free Macromedia Flash player, currently available for (at least) Macintosh, MS-Windows, and UNIX often as a plug-in available for Netscape and compatible browsers.

sxc & stc

A StarOffice (version 6 and later) or OpenOffice spreadsheet document (they are the same); can be read on any machine with StarOffice or OpenOffice installed (available for free for many platforms including WinTel boxes, Macs, and some types of UNIX boxes). The version with the "x" is an ordinary document; the version with the "t" is a template for building other documents.

sxd & std

A StarOffice (version 6 and later) or OpenOffice drawing program document (they are the same); can be read on any machine with StarOffice or OpenOffice installed (available for free for many platforms including WinTel boxes, Macs, and some types of UNIX boxes). The version with the "x" is an ordinary document; the version with the "t" is a template for building other documents.

sxi & sti

A StarOffice (version 6 and later) or OpenOffice Impress document (used for holding presentations -- the two use the same format); can be read on any machine with StarOffice or OpenOffice installed (available for for free for many platforms including WinTel boxes, Macs, and some types of UNIX boxes). The version with the "x" is an ordinary document; the version with the "t" is a template for building other documents.

sxm & stm

A StarOffice (version 6 and later) or OpenOffice math document (they are the same); can be read on any machine with StarOffice or OpenOffice installed (available for free for many platforms including WinTel boxes, Macs, and some types of UNIX boxes). The version with the "x" is an ordinary document; the version with the "t" is a template for building other documents.

sxw & stw

A StarOffice (version 6 and later) or OpenOffice word processing document (they are the same); can be read on any machine with StarOffice or OpenOffice installed (available for for free for many platforms including WinTel boxes, Macs, and some types of UNIX boxes). The version with the "x" is an ordinary document; the version with the "t" is a template for building other documents.

t64

The C64 tape file stores the contents of an entire C64 tape in a single file. It is binary, but is designed to work on any machine that has C64 emulator software (C64 emulators are available for many platforms, including UNIX, Macs, and WinTel boxes) as well as (of course) the C64 (or C128) itself.

tar

Groups of files are often tarred together so that they may be handled as a single file. A file that ends with this extension is probably such a file and will have to be untarred into its individual components before use. The whole concept of tarring started on UNIX machines, but it is also possible to get programs to tar and untar on most other computers.

tcl

A file with this extension is typically a program written in the Tcl programming language. It should be in simple ASCII and should be usable on any machine with Tcl/Tk.

template & tem

A template file written in the C++ programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a C++ compiler. A template file will typically also require some C and h files to be used in any meaningful way.

tfm

The TEX Font Metric file is used to provide information on TEX fonts. It is binary, but will work on any machine that has TEX installed.

tga

The Targa image format can read by numerous different image manipulation programs for several different platforms. It is frequently used in special effects and raytracing work.

tgz

It is so common to tar a batch of files and then compress them with gzip that the result is often given the "tgz" extension instead of "tar" and "gz" in sequence.

thtml & ttml

Very similar to shtml but utilizes Tcl to do its work.

tiff & tif

The tagged image file format is a lossless format for storing image data. TIFF is a popular output format for scanners and other similar devices. Free viewers exist for TIFFs on most platforms, but most browsers lack direct TIFF support.

topc

This is a simple ASCII file used by the TopicCrunch program (available only on MS-Windows) to store keywords plus domain and search engine settings to allow re-running search engine optimization checks.

tr

A TomeRaider e-book file. It is binary and not particularly portable; it can currently only be used with Palm Pilots, Psions, Windows '98, and some flavors of Windows CE. It is currently not supported on any other type of PDA or OS.

tsv

This file type is used to exchange data between numerous spreadsheet and database applications. It is in simple ASCII (in fact, it is just a collection of tab-separated values) and is platform-neutral.

ttf

The Truetype Font format is used for storing vector fonts. It originated on Macintoshes and WinTel boxes and can be made to work on many other systems as well. Unfortunately there are two slightly different Truetype formats in use; modern Macintoshes can use either, WinTel boxes are restricted to the WinTel type, and other systems can sometimes use either but will usually be restricted to one or the other. Free conversion programs exist to convert between the two.

txt & text

This is a generic sort of extension indicating a simple text file (usually ASCII). It can be readily used on virtually any computer, although sometimes character set differences will require slight conversions. A file with this extension will not typically have significant formatting; that would decrease its simplicity and portability.

type

This is a generic sort of extension indicating some sort of "type" data; of particular note though is that native Amiga font files often use this extension.

u00

ulx

A Glulx data file. Typically it will represent an interactive fiction story (or interactive tutorial, or similar). It is binary but will work on any machine with some flavor of Glulx interpreter, and such interpreters are available for several different platforms, usually for free. It is currently not as portable as a Z-machine file, but it is being actively developed so this will probably change in time.

uni

This is a generic sort of extension usually indicating a simple Unicode text file. It can be readily used on virtually any computer. A file with this extension will not typically have significant formatting; that would decrease its simplicity and portability.

url

This file is an Internet short-cut. It is used to indicate a URL and can usually be opened with a browser.

usr

A user-defined file used by C64 or C128 computers for both data storage and some applications. Since they are by definition "user-defined", these can vary wildly in all details. They are heavily used by all Commodore versions of GEOS.

uu & uue

The uuencode program will convert a binary file to ASCII, typically for purposes of e-mailing. A file with this extension has usually been converted in this way and will need to be uudecoded in order to be properly used. The whole uuencoding / uudecoding thing started on UNIX systems, but today free uuencoders and uudecoders are available for virtually every computer platform. Unfortunately it was never really standardized, so sometimes uudecode on one machine will not properly work on something that was uuencoded on another.

vbm

A video bitmap file; a monochrome image format originally designed for the Commodore 128, but readable by many other machines as well with the appropriate software. It is essentially a binary version of the X-bitmap format.

vf

The Virtual Font format is used to store composite font information. It will work on any machine that has TEX installed.

vor

This extension can indicate one of two unrelated things. The more common one is a file pretty much like an sdw, sdd, sdc, or sda file, but a template rather than an ordinary document. The less common is a so-called Vorpal file: a proprietary format designed by Epyx for fast loading from slow devices.

vsd

A Visio document is a binary file used to hold charts and diagrams. It can be read on any machine that has either Visio or OmniGraffle Pro installed.

wav

The waveform audio file format was originally developed on WinTel boxes but can now be played on many platforms. It is one of the most popular audio formats found on the web. It cannot be played on as many different machines as mono au files, though, so for simple voice recording the au format may be preferable.

wdb

A DataViz document; can be read on any machine with DataViz (MS Works) installed (available only for WinTel boxes and Macs, but usually not for free).

wdgt

Unfortunately this extension can mean one of two slightly incompatible things: an Apple Dashboard Widget or an Opera Widget. In each case it'll function a lot like an applet desktop accessory. In the former case it will work on computers running Mac OS X or KDE. In the latter case it will work on computers running the Opera browser. The Apple Widget will consist of a group of files kept together in an application bundle; the Opera Widget will consist of a group of files zipped together. See also the similar widget extension.

widget

This is probably a Yahoo! (formerly Konfabulator) Widget. It'll function a lot like an applet desktop accessory. It usually consists of a group of files zipped together. See also the similar wdgt extension.

wk1

A Lotus 123-2 document; can be read on any machine with Lotus 123 installed (available only for WinTel boxes, Macs, and some UNIX boxes, but usually not for free).

wks

A Lotus 123-1A document; can be read on any machine with Lotus 123 installed (available only for WinTel boxes, Macs, and some UNIX boxes, but usually not for free).

wmf

A Windows metafile is like an ordinary CGM metafile but possibly containing structures that make it specific to MS-Windows. It is not really more capable than the ordinary CGM format, but less portable and therefore less desirable.

wp, wp4, wp5, wp6, & wpd, wpp

A WordPerfect document. It is binary but will work directly on any machine that has WordPerfect.

wps

A Microsoft Works word processing document. It is binary and generally requires Microsoft Works to be read, although some versions of Microsoft Word will read some versions of Microsoft Works documents some of the time.

wra & wr3

A Wraptor file is similar in all respects to an arc file, but a different method of compression was used. Programs to uncompress this type of file are apparently only found on the C64/128.

x3d

An Extensible 3-Dimensional Graphics file contains XML data representing a 3D image. It can be viewed by any application that can understand X3D data.

xbm

The X bitmap format was designed specifically for small icons. It is in simple ASCII and only supports monochrome. It is supported on many platforms and most browsers. It is not space efficient, but because it is always used for small images, this is usually not a problem. Historically it is the original World-Wide Web image format; the others came along later.

xcf

The Experimental Computing Facility format is used by the GIMP to store image files.

xls

An Excel spreadsheet document; can be read on any machine with Microsoft Excel (available only for WinTel boxes and Macs, but usually not for free). Note that in newer versions of Excel Microsoft has split this format into two variants that use either the XLSX or the XLSM extension.

xlsm

This extension is used by one of Microsoft's replacements for the XLS format. It can only be read by machines with MS-Excel (at least version 12) installed. Note that unlike the XLSX variant, these files can contain embedded code (the "m" stands for "macro") and should thus not be considered safe if they come from untrusted sources.

xlsx

This extension is used by one of Microsoft's replacements for the XLS format. It can only be read by machines with MS-Excel (at least version 12) installed. It is a safer sibling to the XLSM format.

xml

An extensible markup language file contains data in simple ASCII that can be read with particular applications on a case-by-case basis. Much of the data contained within the World-Wide-Web is actually in this format.

xpi

An extension package installer used by Mozilla and several related browsers. It will generally perform the install automatically if entered as a URI.

xpm

The X pixmap format was designed specifically for small icons. It is in simple ASCII but supports multiple colors, even a transparent color. It is supported on many platforms and most browsers. It is not space efficient, but because it is always used for small images, this is usually not a problem.

xps

XPS stands for XML Paper Specification and is quite similar in intent to PDF. Being newer, it does not yet have the widespread support that PDF enjoys, though.

xul

An XML user-interface language file contains XML data that can be parsed by applications that understand XUL.

xwd

An X-Windows dump is the X-Windows variant of the bmp and pict image file formats with similar limitations.

Z

The UNIX compress program is used to reduce the size of a single file. If more than one file is to be compressed, they must be tarred together first. By default the compress program will create a file with the "Z" extension. This file will have to be uncompressed before use and will not be easily uncompressed on non-UNIX systems. It is probably better to use gzip both for reasons of portability and compactness.

z1, z2, z3, z3, z4, z5, z6, z7, z8, & zco

A Z-Machine data file. Typically it will represent an interactive fiction story (or interactive tutorial, or similar). It is binary but will work on any machine with some flavor of Z-machine interpreter, and such interpreters are available for virtually every machine in existance (often for free). The 1-4 variants are classic Infocom style; the 5-8 variants are newer. Types 1, 2, 4, 6, and 7 are extremely uncommon; most Z-machine data files are types 3, 5, or 8. Type 8 allows for a much longer, more complex story than the other types -- if types 3 & 5 are viewed as being similar to short stories or novellas, type 8 can be viewed as being as long (or longer) than a full novel. Type 6 provides some graphics support.

zcml

Zope Configuration Mark-up Language files are XML files used for the configuration of Zope Web sites.

zip

A file that has been compressed with either the zip or pkzip program will get the "zip" extension. It is similar in portability and performance to gzip (with gzip being perhaps slightly more portable), and similar in performance (but more portable than) sit. Unlike gzip, zip does not require a separate tarring step; it uses its own method to do the equivalent.

zpt & pt

Used for creating dynamic XML, a page template is itself an XML file built around a handful of namespaces including TAL, TALES, METAL, and I18N. Originally used only with Zope, page templates are now being used with several applications.