Mike Nash, corporate veep of Windows Product Management, called the theories on how Microsoft counted its Windows releases "both a trip down memory lane and quite amusing." Here's his explanation of how they actually did it:

"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."

The stuff about code version numbers and application compatibility is important. Recall how poorly Vista fared in application and driver compatibility in its early months on the market. It was a failure that contributed to the product's poor initial reception, a stain that has remained in much of the popular perception even though the issues are largely in the past.

Nash continues:

"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."

Some observers have inferred from the fact that Windows 7 will actually be version number 6.1 that the forthcoming release, due in 2010, will be less significant.

Not so, Nash says.

"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."

Two things worth noting here:

First, the tone of Nash's conclusion sounds different than the soft-pedaling he gave Windows 7 yesterday when he announced its official name: With the next release, he wrote yesterday, Microsoft is trying "to stay firmly rooted in our aspirations for Windows Vista, while evolving and refining the substantial investments in platform technology in Windows Vista into the next generation of Windows." Compare that with the statement above and tell me what you think.

This speaks to one of the big unknowns that I hope will be resolved at Microsoft's upcoming Professional Developers Conference: How closely will Microsoft position Windows 7 to Vista? The tone and words executives use to describe 7 will be worth watching.

Second, Microsoft appears hell-bent on not repeating the compatibility mistakes made in Vista. So much so that they're naming the OS Windows 7, even though it's technically version 6.1. I wonder what other compromises will be made in the name of compatibility.