Build list legend
|Version of Microsoft Windows|
|OS family||Windows NT|
Neptune was the codename for an unreleased version of Microsoft Windows which was intended to be the first NT-based version to succeed the Windows 9x series. If released, it would have possibly used the version number 5.50, suggested by the file versions from available binaries. The project, however, was scrapped together with Odyssey, a business-oriented release that would have succeeded Windows 2000, and replaced with Whistler in early 2000. As of 2022, only one build has leaked, but others likely exist.
A minor update to Neptune called Triton was also mentioned in internal Microsoft documents from 1998, which was planned to be released 12 months after the initial release of Neptune in March 2001. Neptune would also receive five Service Pack updates during the development of Triton.
Although only build 5111.1 is currently available, there is some evidence for other builds as well, mostly from file versions in various Windows Me builds.