Classic Mac OS

Classic Mac OS is a general term used to describe vintage Macintosh system software (Mac OS) before (and not including) Mac OS X. PowerPC-based Macs running versions of Mac OS X up to 10.4 Tiger are able to employ the Classic environment, providing compatibility with "Classic" applications, provided they can function on Mac OS 9. Macs using Intel and Apple Silicon processors are unable to run "Classic" software and therefore must employ an emulator.

Release history
Major releases of the classic Mac OS:
 * System 1, released January 24, 1984
 * System 2, released April 1985
 * System 3, released January 1986
 * System 4, released January 1987
 * System 5, released October 1987
 * System 6, released April 1988
 * System 7/Mac OS 7, released May 13, 1991
 * Mac OS 8, released July 26, 1997
 * Mac OS 9, released October 23, 1999