Filename Extensions

When you double-click on a document, the computer will open the correct program so you can see the document. But how does the computer know which program to open? Well, it does that by looking at the extension (suffix) to the file name. An extension is a period followed by three (or more) letters and/or numbers.

Most computers do not show the extensions, because some people are confused by them. However, if they are hidden, they can cause problems. For this class, you should make extensions VISIBLE. Hopefully, you can do this on your home computer as well; it will be helpful when you make web pages.

Here is how you can make the extensions visible or invisible: in any file and folder window, go to the TOOLS menu, and choose FOLDER OPTIONS...

You will see a dialog box like this:

In the dialog box, click on the VIEW tab, and then look in the box below. Find the choice titled "Hide extensions for known file types." Make sure that the box next to it is NOT CHECKED. Click "OK." Now your extensions will be visible. Do the same thing in reverse to make them hidden again.

Here are some common extensions, with identification:

When the extensions are visible, then you can change them yourself, just by editing the filename. But be careful--just changing the extension WILL NOT change the file TYPE! You will only be telling the computer to use a different program to open the file. If you take a Word file and change ".doc" to ".mp3", you will not change your essay into a music file.

The computer usually "knows" which extensions open which programs. However, there is a way to "teach" your computer to use different programs for various extensions. For example, you can tell your computer to automatically open ".mp3" files with Windows Media Player, or with iTunes, or with any other audio program. To do this, right-click on a file of the type you want to open. Choose "Open with..." from the pop-up menu. If you are given another submenu, then select "Choose Program..."

This dialog box will appear:

You can choose the program you want from the list. Check the box which says: "Always use the selected program to open this kind of file." Then click "OK." After you do this, the computer will connect that filename extension with that program. In this example, I connected the ".jpg" extension with the program "Adobe Photoshop."