Lesson 02 | 09 of 13

File Extensions

A file extension is the three letters of a file name after the dot. It is created by the application when you save a file. It is the way operation systems identify which application to use to open a file. Windows often doesn't list them when you are searching for a file, but they are there. Examples are: .txt, .doc, .exe, .html, .jpg, .gif, .wav.
However, sometimes you will want to save a file with different extensions. In order to do that, select Save As in the File menu. The Save As box will open. Choose from the drop-down menu next to Save As Type the type you want to save. The extension of the type will be created for you by the application.
Here is an example from Microsoft Word:

  • Some File Extensions:
  • Graphic files: .bmp, .gif, .tif, .jpg
  • Word document: .doc
  • Program file: .exe
  • Unformatted text: .txt
  • Excel spreadsheet file: .xls
  • Compressed file: .zip
  • Rich Formatted Text: .rtf
  • Sound file: .wav
  • Acrobat Reader file: .pdf


Knowing what extensions certain file types use can be helpful.
While most filename extensions consist of three characters, they can be anywhere from one to five characters long. When you double-click a file, your computer uses the file's extension to determine what program should open it. It is possible to change file's extension, which will also change the program your computer uses to open the file. Be careful when changing a file extension, since it may cause your computer to be unable to open the file. For example, if you change a file with a .txt extension to a .doc extension, Microsoft Word should open it. However, if you change a .txt file to a .jpg file, the file will not open.

Since there are tens of thousands of software programs available, there are also tens of thousands of filename extensions. While it is a good idea to learn the most commonly used file extensions, it is not possible to remember them all.