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Windows XP Pro: The Missing Manual, Second Edition by L.J. Zacker, Craig Zacker, David Pogue

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Filename Extensions

Every operating system needs a mechanism to associate documents with the applications that created them. When you double-click a Microsoft Word document icon, for example, Word launches and opens the document.

In Windows, every document comes complete with a normally invisible filename extension (or just file extension)—a period followed by a suffix that’s usually three letters long. Here are some common examples:

When you double-click this icon this program opens it
Fishing trip.docMicrosoft Word
Quarterly results.xlsMicrosoft Excel
Home page.htmInternet Explorer
Agenda.wpdCorel WordPerfect
A home movie.aviWindows Media Player
Animation.dirMacromedia Director

Tip

For an exhaustive list of every file extension on the planet, visit http://whatis.com; click the link for “Every File Format in the World.”

Behind the scenes, Windows maintains a massive table that lists every extension and the program that “owns” it. To see this list, choose ToolsFolder Options from the menu bar of any folder window. When the Folder Options box appears, simply click the File Types tab (Figure 5-8).

Displaying Filename Extensions

It’s possible to live a long and happy life without knowing much about these extensions. Indeed, because file extensions don’t feel very user-friendly, Microsoft designed Windows to hide the suffixes on most icons (see Figure 5-9). If you’re new to Windows, and haven’t poked around inside the folders on your hard drive much, you may never even have seen them.

Figure 5-8. Each ...

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