Within a single directory, filenames must be unique. However, two files in different directories may have the same name. Some operating systems, such as UNIX and the Macintosh operating system, allow a file to have more than one name, called an alias. (In Unix, aliases are called links or symbolic links.)

See the Data Formats and Their File Extensions page in the quick reference section of Webopedia for a list of file extensions and the programs they coincide with.