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 .

u00: 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 user-defined file.
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.