Microsoft has confirmed that the next version of its operating system will be called Windows 7. But the decision has invoked surprise and confusion among some observers.

The announcement was made in a blog posting by Mike Nash, Microsoft's corporate vice president of Windows, who explained that the codename of the operating system would remain when the product is officially released.

"While I know there have been a few cases at Microsoft when the codename of a product was used for the final release, I am pretty sure this is a first for Windows. You might wonder about the decision," Nash wrote.

The name was chosen to keep things simple, Nash explained. "This is the seventh release of Windows, so therefore 'Windows 7' just makes sense," he said.

Nash added that Microsoft had decided against using an aspirational name because the company's aspiration lay with Windows Vista.

Windows 7 will represent an evolution and refinement of the "substantial investments" made in the Windows Vista technology, he said.

But responses to Nash's blog suggest that Microsoft may not be keeping things simple after all. A number of commentators point out that the release is not Microsoft's seventh at all, whichever way you look at it.

According to the general consensus, there have already been seven versions of Windows: Windows, Windows 2, Windows 3.0, Windows NT, Windows 2000, Windows XP and Windows Vista. This new edition should therefore be Windows 8. If Windows XP is not counted because it is Kernel 5.1, then 'Windows 7' should be Windows 6.1.

"What will happen when the 'real' Windows 7.0 comes around in x years? Wait a second I thought Windows 7 was released years ago," said a posting under the name of 'PatriotB'.

"I guess you guys thought Windows 6.1 didn't slip off the tongue. But still, don't lie to people and muddy everything up."

Another reader, called 'Resplendent', said: "It does seem like an odd shift to go from 'names' (Millennium/XP/Vista) back to numbers again."

Further details of Windows 7 will be released at Microsoft's Professional Developers Conference and WinHec events, at which Microsoft will share a pre-beta developer only release with attendees.