This means that all DOS filenames have two parts, with up to 8 and 3 characters respectively. The first part is also often referred to as the filename, which can be a little confusing, but there you have it. This part can have anywhere from 1 to 8 characters in it. This is followed by the extension , which can have from 0 to 3 characters. Separating these two parts is a period, but in computer language we never call it a period, we call it a dot.
Filenames can use any letter of the alphabet or any numeral.
Many of the other characters found on a keyboard are also allowed, but not all of them. Finally, it does not matter whether you use upper case or lower case letters. File Extensions A file in DOS does not need to have an extension. You can test this quite easily by creating a file called FOO and saving it in your DOS test directory. Do a directory listing using the DIR command, and you will see it listed.