I was rather confused when I first heard that Microsoft was going to call the new version of Windows for Windows 7 since in my mind it’s Vista that should have been version 7.
The first version of Windows NT got the version number 3.1 to align with the 16-bit version of Windows. Later there where a 3.5 and a 3.51 version of NT before the release of NT 4. After that came Windows 2000, which was Windows 5. Then Microsoft released XP which I thought should have been version 6, but the version number was set to 5.1 even though it was very unlike Windows 2000.
Instead Vista got the version number 6, even though I think it should have been 7. Now Windows 7 release candidate is available for download. The main difference between Vista and Windows 7 is the new taskbar and not much more (well, there are a bunch of other goodies under the hood, but from a user perspective the taskbar is the main noticeable difference) so in a sense it should really be Windows 7.1 since I think Vista should have been version 7. But lo and behold, the version number in Windows 7 is 6.1 and not 7.