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 .

p00 - p99: 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 program file.
pages: An Apple Pages document. It is a compound document built from XML files (and various types of media files) and will work on any machine that has Apple Pages installed.
pas: A source file written in the Pascal programming language. It should be in simple ASCII and (depending upon how portably it was written) should be usable on any machine with a Pascal compiler.
pbm: The portable bitmap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. It is similar to pgm but can only handle monochrome images. There is also a binary variant that is more space efficient but less portable.
pcf: The Portable Compiled Format is a format for storing bitmapped fonts. It is binary, but still designed to be portable. It originated on the X-Windows system but can be made to work on other systems as well.
pdb & prc: Probably a Palm package of some type, either a Palm Database or a Palm Resource. In either case, it is basically a program made to run on the Palm platform, and will sometimes be a Palm DOC type of e-book. It is binary and not particularly portable, but applications exist to read Palm DOCs on a few other platforms.
pdd: The PhotoDeluxe Data image format was developed by Adobe for use with its PhotoDeluxe program and can be read by any machine that has it installed (it is only available for Macs and WinTel boxes, and then generally not for free).
pdf: The Portable Document Format is a format for storing all manner of documents. PDF documents can contain sounds and animations in addition to simple images and are binary in nature. PDFs are also fully capable of containing hypertext, but unlike HTML will guarantee proper printout. It is a product of Adobe and free PDF interpreters are available for most machine types. There are also free PDF plug-ins available for Netscape and compatible browsers.
pfa: The PostScript Font ASCII is a format for storing vector fonts. It is simple ASCII and originated on the X-Windows system. It is a creation of Adobe and the ASCII counterpart to the less widely recognized PFB.
pfb: The PostScript Font Binary is a format for storing vector fonts. It is binary but designed to be portable. It originated on the X-Windows system. It is a creation of Adobe and the binary counterpart to PFA.
pfm: The Printer Font Metric file is used to provide font information on an MS-Windows machine. It probably will not be useful anywhere else.
pgm: The portable graymap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. It is similar to ppm but can only handle shades of gray. There is also a binary variant that is more space efficient but less portable.
php, php3, & php4: Very similar to shtml but utilizes PHP to do its work.
phtml: Very similar to shtml but utilizes Perl to do its work.
pict, pct, & pic: Macintosh bitmap picture format. This is a fairly simple bitmap format that can be viewed on many different platforms but is directly supported by very few browsers. It is not used too much on the web with GIFs, JPEGs, PNGs, and even X-bitmaps being preferred. It is somewhat similar to the bmp format, although utilizes compression and so boasts smaller file sizes.
pif: This extension can be used for more than one thing. First is the process interchange format, a temporary file used to facilitate communication between different programs. There is also a rarely seen picture interchange format which can be used to store images. These two formats are both binary but can be used on pretty much any platform that has the appropriate support. The third possibility is a Microsoft program information file; it can be used only on MS-Windows machines and is in reality just what is more commonly called an alias, link, or shortcut.
pk & pxl: The TEX Packed font format is used for storing bitmapped fonts. It is binary but will work directly on any machine that has TEX. The PXL form is obsolete; all current files should be PK.
pkg: Possibly a Newton package of some type; a program made to run on the Newton platform. It will frequently be a Newton book type of e-book or a Newton Script program. It is binary and not particularly portable, but applications have been written to read Newton books on a few other platforms. Note that somewhat unfortunately, this extension is also used for the unrelated (and now more common) Apple package format used on Macs.
pl: This extension will typically mean one of two completely different things. The more common is a Perl script, a program written in the Perl language. It should run on any machine that has Perl installed and is simple ASCII. The second is a TEX Property List file. It is used to provide information on fonts. It is also simple ASCII, so it should not be too hard to tell the two apart.
pm, pm3, pm4, & pm5: An Adobe PageMaker document. It is binary but will work directly on any machine that has Adobe PageMaker (currently only available for Macs and WinTel boxes).
png: The portable network graphics image format is designed to replace the GIF. In fact, PNG is sometimes jokingly said to really stand for "PNG's not GIF". PNG is completely lossless and can handle millions of colors; it is not limited to a palette of two-hundred fifty-six like GIF. It also has full support for transparent colors. Its only real disadvantage is that right now not all browsers support it directly. It has been recommended by the W3C, though, so odds are pretty good that most (if not all) future versions of browsers will provide direct inline support for PNG, and PNG images will start to become more commonplace on the web.
pov: POV-Ray source files will typically be given this extension. POV-Ray source files are simple ASCII and should be readable on most platforms. Effective use can only be made of them on machines with POV-Ray, however.
ppm: The portable pixmap format is an ASCII image format designed primarily to be portable across different platforms. It is not particularly space efficient, but can be handled by many different image manipulation programs. There is also a binary variant that is more space efficient but less portable.
ppt: A Microsoft PowerPoint document. It is binary but will work directly on any machine that has Microsoft PowerPoint or Apple Keynote. Note that neither of these products is free, so this is not a good distribution format. Also, Microsoft has split this format into two different variants (one uses the PPTX extension while the other uses the PPTM extension) in newer versions of PowerPoint.
pptm: This extension is used by one of Microsoft's replacements for the PPT format. It can only be read by machines with MS-PowerPoint (at least version 12) installed. Note that unlike the PPTX 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.
pptx: This extension is used by one of Microsoft's replacements for the PPT format. It can only be read by machines with MS-PowerPoint (at least version 12) installed. It is a safer sibling to the PPTM format.
pqa: A Palm query application is used to relate a group of Palm Database files together.
press & pre: Press files are typically source files for the Newton Press program and will work on any computer that has Newton Press (currently available only on Macs and WinTel boxes). Newton Press can save press files as Newton books.
prg: A program file is a program usually for a C64 or C128 computer, although it could also be for something else. It will probably not work on a platform other than the one for which it was designed.
ps: Postscript files are capable of holding quite a variety of information ranging from pictures to text documents to font definitions. Postscript files can be directly interpreted by many printers, and Postscript viewing software comes with many computers and can be obtained for free for most others. The most popular free Postscript viewer is called ghostscript and may be downloaded from many different FTP sites. In truth, Postscript is actually a full language, and Postscript files can perform all manner of things.
psd: The PhotoShop Data image format was developed by Adobe for use with its PhotoShop program and can be read by any machine that has it installed (current versions are only available for Macs and WinTel boxes, and then not for free).
pxr: The Pixar image format was developed by Pixar and can read by numerous different image manipulation programs for several different platforms.
py: Usually a Python script, a program written in the Python language. It should run on any machine that has Python installed and is typically in simple ASCII.
pyc & pyo: Usually a compiled Python script (that is, Python bytecode), a program written in the Python language. It should more or less run on any machine that has Python installed, but unlike py files will probably be binary rather than simple ASCII. In the case of the "pyo" variant, the script is not only compiled but optimized.
pyd: Usually a Python shared library. It is binary and not portable.