Filename Extensions / Suffixes

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. For example: "My Essay.doc"

How to See Extensions

Most computers do not show the extensions, because some people are confused by them, and might erase them by accident. However, if they are hidden, they can cause problems in other ways. 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.

However, be careful to make them invisible again before you let other family members use the computer! (This should not be necessary if you use different User Accounts.)

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.


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. Instead, you are only telling the computer to open the document with Windows Media Player instead of Microsoft Word.

Common Filename Extensions

Here are some common extensions, with identification:

.exe a program; Windows will try to open it.
.txt a "plain text" or "text only" file (no formatting)
.rtf a "rich text" file (contains formatting)
.doc a Microsoft Word document
.xls a Microsoft Excel document
.ppt a PowerPoint document
a web page document
Image (photo) documents
Movie (video) documents
[Note: sound files can be saved under some movie filename extensions; for example, ".mov" and ".mp4" could be sound-only.]
Audio (sound/music) documents

These are not all the possible extensions--in fact, there are hundreds of different ones. Often, each software application may have its own unique document types.

The Default Application List

Every computer has a list of extensions paired with the software which will open it. When you begin to use a new Windows PC, for example, music/audio and movie files will automatically ("by default") open in Windows Media player; text files will open in Notepad; HTML files will open in Internet Explorer, and so on.

However, you can change these assignments so that certain filename extensions will open whatever programs you wish. Often, when you install new software, it will ask you if you want the new software to be the "default application" for certain file types. For example, if you install iTunes, it will ask you if you would like iTunes to be the default music/audio player, instead of Windows Media Player. If you say "yes," then iTunes will change your filename extension list so that audio files (e.g., MP3, WAV, AAC, etc.) will open iTunes; all such audio icons will change to iTunes document icons.

You can also change default applications by yourself. First, choose one document icon which has an extension you'd like to change the program for. Right-click on it, and 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."

After you do this, that filename extension will continue to be associated with the new program.