Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The original version was the integral and unnamed system software first introduced in 1984 with the original Macintosh, and referred to simply as the System software. This is credited with popularizing the graphical user interface concept. Mac OS releases have existed in two series, Classic Mac OS from 1984 to 2000 and Mac OS X from 2001 to present. Both series share a general interface design, but have very different internal architectures.