While there has always been a Macintosh aftermarket for mice and other pointing devices with two, three, or more buttons, and extensive configurable support (usually through keyboard emulation) to complement such devices in many major software packages on the platform, it wasn't until Mac OS X shipped that support for multi-button mice was hardcoded. X Window System applications, which Mac OS X can also run, have been developed with the use of two or three-button mice in mind.
