Glossary of Computer File Extensions

Filename Extensions List

So you see a file somewhere on the 'net with a name like "" 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 .

r00 - r99
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 relative file. Note that relative files are generally quite rare, and some lesser emulators may not be able to handle them.
The rar format is used for archival purposes.
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.
A Rocket e-book file. It is binary and not particularly portable; in fact it is geared primarily toward dedicated e-book hardware.
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.
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.
A RealMedia movie file will typically store a movie clip. It's generally supported by many different platforms.
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.
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.
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.
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.
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.