A filename extension is a suffix to the name of a computer file applie to indicate the encoding convention (file format) of its contents. In some operating systems (for example Unix) it is a convention, while in some others it is a requirement. Some operating systems limit the length of the extension (such as DOS and OS/2, to three characters) while others (such as Unix) do not. Some operating systems (for example RISC OS) do not use file extensions.