Mike Nash has posted an explanation of “why 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.
That brings us to Windows Vista, which is 6.0. So we see Windows 7 as our next logical significant release and 7th in the family of Windows releases.
We learned a lot about using 5.1 for XP and how that helped developers with version checking for API compatibility. We also had the lesson reinforced when we applied the version number in the Windows Vista code as Windows 6.0-- that changing basic version numbers can cause application compatibility issues.
So we decided to ship the Windows 7 code as Windows 6.1 - which is what you will see in the actual version of the product in cmd.exe or computer properties.
There's been some fodder about whether using 6.1 in the code is an indicator of the relevance of Windows 7. It is not.
Windows 7 is a significant and evolutionary advancement of the client operating system. It is in every way a major effort in design, engineering and innovation. The only thing to read into the code versioning is that we are absolutely committed to making sure application compatibility is optimized for our customers.