Answer: Windows 2000 64-bit

While many projects pride themselves on clever and interwoven codenames, the Windows development team has always taken a more random approach to codenames.

Windows NT 3.5, Windows 95, Windows NT 4.0, and Windows 98 were all named after cities (Daytona, Chicago, Cairo, and Memphis, respectively), and the later releases of Windows XP Media Center Edition were called Harmony and Symphony.

But despite those micro-patterns here and there in the development timeline, there’s no overarching theme to the codenames. Lack of continuity among codenames, then, is as much the pattern as anything else that might emerge from reviewing old Windows codenames. Among all the Windows releases, for example, there is one unique codename standout. Only one Windows desktop/server release has ever been named after a god. Windows 2000 64-bit’s code name was, fittingly, Janus–the Roman god of beginnings and transitions.

There was a planned release of Windows, based on Windows 2000, with the code name of Windows Neptune, but it was never actually released.