Why is it called Windows 10 and not Windows 9?

Windows 10 finally has an official release date — July 29, 2015 — and development is nearing its end, although the latest Insider Preview Build 10122 clearly still needs some work. Let’s take a step back for a moment and address one of the most confusing things about the next version of Windows.

When Microsoft announced its newest operating system last year, the surprise was not that it was coming, but that Windows would be skipping 9 and heading straight to 10. When asked about Windows 10’s name, Microsoft never gave a clear answer. So why, exactly, is Windows 10 getting the nod instead of 9?

Version numbers, schmersion numbers

Windows 8.1: Actually version 6.3, build 9600.

You may remember that between Windows 3 and Windows 7, Microsoft designated each version with a name instead of a number: 95, 98, NT, Me, 2000, Vista, and so on.

 When the company announced Windows 7, there was actually a similar amount of disbelief; after a series of named versions of Windows, it seemed odd to switch back to numbers.

There’s also the fact that the name of each Windows release doesn’t actually match the real version number. For example, Windows 8.1 is actually version 6.3 of Windows. Windows 10 is version 6.4. The last time the release name actually matched the version number was the enterprise-focused Windows NT 4.0, which was released back in 1996. Windows 2000, which was called NT 5.0 during development, was actually version 5.0. Windows XP was version 5.1. Windows Vista was 6.0, Windows 7 was 6.1, Windows 8 was 6.2, and Windows 8.1 is version 6.3.

Windows RT, which only ran Metro apps, was a new and separate beast, but it still sat on top of the core Windows NT kernel. It’s dead now. Technically, modern versions of Windows are still based on the Vista kernel and code base — including Windows 10, which is actually Windows 6.4.

There will be some confusion if (or when) we eventually reach internal version 7.0, but we’ll cross that bridge when we get there.

Alternative theories for skipping Windows 9

First, an ExtremeTech reader called Benny sent an email to say that the number 9 is considered unlucky in Japan. Microsoft has a big enough presence in Japan that it may have skipped Windows 9 to avoid any weirdness or ill will. Benny says that Trend Micro — a Japanese company — did the same thing a few years ago when it skipped version 9 of its antivirus software.
Second, someone purporting to be a Microsoft developer posted this comment on Reddit:


As dumb and yet amazing as this sounds, it is actually quite feasible that there are still a lot of legacy Desktop apps that use this method (or something similar) to check for Windows 95 or 98. Bear in mind that this is just an example piece of code — some developers will check for the OS name (“Windows…”), some will check for the version number (as discussed in the previous section of this story), and some may use other methods entirely to find out what OS the app is running on.