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 .

d00 - d99: 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.