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Quick Definition

An extension is part of a filename that is used to distinguish between different types of files. To find the extension, look for the last period in the file's name; the extension will be everything that comes after that period.

For example, in the filename "mydocument.txt", the extension is the letters "txt" . This is a common extension for a file containing plain text.

Some operating systems can be set so that extensions are hidden from the user. This simplifies things, as typical users don't need to change them or pay attention to them. Changing an extension will change how the operating system treats the file, so it may make the file unusable.

Old file systems limited extensions to a maximum of three letters. Because of this, most extensions are kept very short by convention.