Saugus.net: Glossary of Computer File Extensions

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 .

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.