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File Extensions : A Beginner's Guide

by Oscar Sodani
February 24, 2003

Oscar Sodani is a founder of Help2Go and owner of Help2Go Networks, an IT consulting firm in the Washington D.C. area. Oscar holds the CISSP certification as well as industry certifications from Microsoft, Cisco and Novell.

Have you ever wondered WHY filenames end with a dot and a three letter designation like DOC? These three-letter designations, called file extensions, are everywhere in the world of computing. And they control a lot more than you think...

With this guide, our aim is to show you:

  • Why file extensions were used in the first place
  • How they are used nowadays in different operating systems
  • How to change what file extensions do in
    Windows 95/98/NT
  • The future of file extensions

The REAL dot Com

File extensions are as old as disk operating systems themselves. Besides just naming a file descriptively, like "MYLETTER", early computer users wanted to designate what a file DID as well. On old mainframes, people ended their word processing files with .TEXT, their mail files with .MAIL, and their programs with .EXEC (stands for "executable").

Then came DOS. Microsoft's landmark operating system required that executable programs end their files with .EXE -- so WordPerfect's program file was WP.EXE and Lotus 1-2-3's was 123.EXE. Files that were part of DOS itself ended in .COM, way before the average person had heard anything about the Internet.

Applications got in the act as well. WordPerfect saved your files with a .DOC or a .WP extension. Lotus 1-2-3 differentiated its files by the .WKS extension. Each program came up with its own three-letter extension, and the world was good.

Windows, Macs, and the UNIX way

Even though DOS made file extensions a computing standard, not all operating systems followed suit. The Macintosh Operating System (MacOS) is unique in that each file you create has an extra few bytes dedicated to recording the type of file it is. If you create a letter to your mother with ClarisWorks on a Mac, you can name that file whatever you want. No file extension is needed, because somewhere embedded in that letter, it is written that this file should only be opened with ClarisWorks. Easy -- the way a Mac should be. This doesn't stop you from renaming the file and adding on an extension however. In fact, compressed files often have a .sit or .hqx extension.

UNIX systems don't use extensions by default, either. You can type whatever you want as the name of the file, but that won't change what it does. Files in UNIX are either executable or text. There's no in between, so there's no reason to worry about file extensions. Of course, the exception to the rule exists here also. Compressed files in UNIX are often given .z or .gz extensions, depending on the compression program used.

Bill Gates and his Microsoft crew went another way. They LOVE extensions, and extensions are a very vital part of Windows 95/98 and Windows NT. For the most part, extensions work the same way as they did with DOS, only with more standardization. Most word-processing files end with .DOC; HTML files, including the one you are reading right now, end with .HTM or .HTML; and Excel identifies its spreadsheets with a .XLS extension.

The big difference is that every time you install a new application, that application registers a file extension with Windows. When you install the Microsoft Internet Explorer web browser (IE), IE registers that .HTML extension. From that point on, when you double-click on an .HTML file, Internet Explorer opens that file automatically. The file's normal "blank paper" icon even changes to an Internet Explorer icon instead. Let's say that later on, you install the Netscape Navigator web browser. Upon installation, Netscape will register the .HTML extension. All those .HTML files with the IE logo will now change to have a Navigator icon instead. And it will be Navigator that opens the file when you double-click on it.

Luckily, you can exercise control over what files with particular extensions do when you double-click on them. You can change this registration by opening up Windows Explorer and clicking on the "View" menu. Choose "Folder Options" and a dialog box will appear. Click on the tab marked "File Types" and you will see a list of every registered file extension on your computer. Feel free to edit them as you wish -- this is your PC, and extensions are supposed to do what YOU want them to do. For example, if your friend works on a Macintosh, and consistently sends you documents with a .LET extension (for letter), you can manually add the .LET extension to your system and associate it with MS Word.

The Dying Extension

According to recent material released by Microsoft, extensions won't be around for too much longer. In Windows 2000 and XP, Microsoft has tried hard to hide file extensions from their users. In upcoming versions of Windows, files will contain extra bytes that define which application they belong to (just like the Macintosh). Extensions will not be needed anymore, and people can name their files whatever they want and not have to worry about those funny three letters at the end. Good or bad, this change is coming, so I hope that Microsoft follows through on the promise.

With so many applications, the number of extensions has gotten out of hand. For more information, consult the File Extension Index at, which lists all the file extensions in use!

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