"There's been a lot of lively discussion since I confirmed yesterday that the official name for the next version of the Window client operating system will be "Windows 7" about how we got to the number "7."
I'll say up front, that there are many ways to count the releases of Windows and it's been both a trip down memory lane and quite amusing to read all the different theories about how we got to the number "7."
Anyway, the numbering we used is quite simple. The very first release of Windows was Windows 1.0, the second was Windows 2.0, the third Windows 3.0.
Here's where things get a little more complicated. Following Windows 3.0 was Windows NT which was code versioned as Windows 3.1. Then came Windows 95, which was code versioned as Windows 4.0. Then, Windows 98, 98 SE and Windows Millennium each shipped as 4.0.1998, 4.10.2222, and 4.90.3000, respectively. So we're counting all 9x versions as being 4.0.
Windows 2000 code was 5.0 and then we shipped Windows XP as 5.1, even though it was a major release we didn't' want to change code version numbers to maximize application compatibility."