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 .

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.
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.
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.