What is a file extension?

A file extension is the last part of a file name, and is everything after the dot.

For example, consider the file name "readme.txt". The file extension here is ".txt"

The file extension is used to indicate the type (or purpose) of the file. The operating system (such as Windows) looks at the file extension when determing what application should be used to open the file.

For example, the file named "readme.txt" has the file extension of "txt" which indicates that the file is a text file (and can be opened and viewed using any standard text editor).

Sometimes a file extension is referred to including the dot, for example ".txt". Including the dot as part of the file extension serves to make it more obvious that it is a file extension, but there is by no means universal agreement over whether a file extension should be written with or without the dot.

You might also be interested in ...

You might find the following related FAQ articles of interest:

• What is a file type?
• Are file extensions always 3 letters?
• What if you cannot see the file extension?