Are file extensions always 3 letters?

No. Whilst the majority of file extensions are 3 letters long, they do not have to be. A file extenion can be a single letter (or digit) or many letters (or digits).

However, this was not always the case. Back in the early history of computing, in the 1960s, Digital Equipment Corporaton (DEC) introduced the concept of a file extension into their operating systems. Originally the file name was limited to up to eight characters and the file extension to three characters. This was then copied by later operating systems, including MS-DOS (MicroSoft Disk-Operating-System) the forerunner to Microsoft Windows. MS-DOS limited file names to up eight characters with up to 3 characters for the file extension. This became known as the 8.3 file convention - 8 characters for the filename and 3 for the file extension. This 8.3 restriction was removed in advances in different opreating systems, including early versions of Windows.

These days it is unusual for an operating system to place any restriction on the number of letters for the file extension. Indeed it is would be unusual for a modern operating system to store the file extension separately (as was the case with many early operating systems). Because of this the size restriction on the length of the file extension has been removed and file extensions can be as long or as short as required.  The concept of a file extension has remained as it provides a simple means of identifying the purpose of a file.

You might also be interested in ...

You might find the following related FAQ articles of interest: