How Does Microsoft Count to (Windows) 7?

On the eve of the Windows 7 launch, we've been doing some reminiscing, but it hit us: There have been way more than six uniquely named prior versions of Windows. So how does Microsoft count?

1, 2, and 3: These were the original DOS-based Windows that started it all. Windows 3.1 is the first version I remember (and even that only vaguely).

4: Windows 4.0 includes Windows 95, 98, ME and NT 4.0.

5: Windows 2000 and all the flavors of Windows XP.

6: The oft-maligned Windows Vista (and Windows Server 2008).

And that brings us to Windows 7. What a long, strange trip it's been. [Wikipedia]