Type code

A type code is a mechanism used in pre-OS X versions of the Macintosh operating system to denote a file's format, in a manner similar to file extensions in other operating systems. Codes are limited to four characters. For example, the type code for a HyperCard stack is STAK; the type code of any application program is APPL.

Type codes are not readily accessible for users to manipulate, although they can be changed with certain software tools.