Filename Extension

Quick example: the .doc part of my_paper.doc

Filename extensions are some operating systems' ways for remembering what program should open what kind of file. For example, when you install Microsoft Office it tells Windows to remember to always use Microsoft Word to open files whose names end in .doc. These are known as filetype associations.

Files in Windows must have a filename extension; Windows doesn't have any idea what to do with a file lacking one. Mac OS X, on the other hand, is usually pretty good about figuring out what kind of file something is without the aid of a filename extension.

Note: In later releases of Windows XP, displaying known filenames extensions is disabled by default. To force XP to always show extensions, click on My Computer, go to Tools > Folder Options... > View tab, and uncheck Hide extensions for known filetypes.

Choosing which programs to associate with which extensions

Sometimes, you'll want to use a different program to open the file than the program that is selected by default. To do this, right-click on the file and select Open with. You'll then get a list of other programs usually associated with this file type, as well as the option to choose another program. In the example below, Windows offers to let me view the .jpg picture file in four different programs. If I wanted to use something not on the list (say, Photoshop), I can select Choose Program... to do so.