Microsoft’s new operating system launches on Thursday and you may be asking: How did we get to Windows 7? Did I miss 5 and 6?

No, you didn’t. But Microsoft Corp.’s names for the successive versions of Windows have been more than a little confusing. It’s easy to get the impression that with every new version of Windows, Microsoft wants us to forget that there was a previous one.

Long ago, we had Windows 1, 2 and 3. So far, so good. Then Microsoft started naming its consumer software after the year of release, like a car, and we got Windows 95. That was followed by 98, while professional users got 4.0. But Windows 2000 wasn’t for consumers at all - the professional version was now named for its vintage as well.

The new millennium threw a spanner in the year-numbering works. Microsoft balked at naming its new system “01.” Naming it “2001” wouldn’t have worked either: imagine all the jokes about the homicidal computer in the movie “2001: A Space Odyssey.” So the new Windows became “XP,” a not entirely self-evident contraction of “experience.”

There didn’t seem to be any other two-letter combination handy to capture Microsoft’s goals for the next Windows, so it became “Vista.” A vista is what you might see out of a window, so that makes sense, right? Someone thought so.

Vista bombed, prompting Microsoft to make another clean break - the third one - and give us Windows 7.

Microsoft’s official rationale is that “7” is the seventh version of Windows. It gets there by counting up from Windows NT 4.0, skipping Windows 98 and counting both XP and 2000 as No. 5. Vista was No. 6.

Adding to the confusion, Steven Sinofsky, Microsoft’s president of Windows, has a variant explanation, saying that Windows 95 was the fourth version of Windows. But Windows 7 is descended from NT 4.0, not Windows 95.

And to further complicate matters, Windows 7 is really Windows 6.1. That’s what the operating system will tell software applications that are trying to check which version of Windows they are running on. Windows 7 will say it’s 6.1 because it’s really a small upgrade from Vista, and programs designed to run on Vista should run with no problems on 7.

“The decision to use the name Windows 7 is about simplicity,” according to Mike Nash at the official Windows blog. He then lays it out in terms as clear as the vista from a newly polished window.

Coming up with a new “aspirational name” like XP or Vista, he writes, would “not do justice to what we are trying to achieve, which is 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.”

John Long, a retail strategist at consulting firm Kurt Salmon Associates, points out that it’s difficult to come up with words like “Vista” that work well in all languages - an important consideration for a world-spanning product like Windows.

“Going back to numbers is logical,” Long said. He also pointed out that even if Microsoft has been indecisive about what to stick after the “Windows,” it has at least been faithful to the “Windows” brand.

If you’re going to jump on the number train, seven is a good place to do it. The number has mostly positive connotations, if you exclude the “Seven Deadly Sins,” Long said. In Japan in particular, seven is an auspicious number. Images and statues of the seven gods of happiness and luck are commonplace.

Contrast that with Apple Inc.’s approach. It used a consistent numbering scheme for versions 1 through 9 of its operating system. When it got to 10, the current version, it started adding the names of big cats to the sub-releases: Cheetah, Puma, Jaguar, Panther, Tiger, Leopard and Snow Leopard. Each one tells you there’s a purring beast inside, ready to tear into your tasks, and each one sounds consistent with the last.

“I think Apple to some degree likes to play the underdog and likes to imbue their products with a lot more personality,” Long said. “Animals do that quite effectively.”

The Windows franchise faces a lot of challenges - among them, Internet search leader Google Inc.’s plans for its own operating system. So Windows may find itself the underdog one day.

By then, Apple will have already had a lock on cats, but Microsoft could turn to dogs: Windows Greyhound (it’s fast), Windows Dachshund (it’s compact, good for small computers) or Windows Cocker Spaniel (pretty interface).

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