Why is it called Windows 10 and not Windows 9?

Windows 10 logo: Windows 9 kinda

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

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.
Windows 8: Actually version 6.3

Windows 8.1: Actually version 6.3, build 9600.
There’s also the fact that the name of each Windows release doesn’t actually match thereal 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:
Reddit comment explaining why Windows 9 was skipped
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.
What’s in a name?
Ultimately, Windows 10 is just a name. Windows 9 probably would’ve made more sense — and it’s going to cause some grief with novice users who just don’t understand what happened to Windows 9. But Windows 10 isn’t any more right or wrong than calling Vista’s successor Windows 7.
Why Windows 10 wasn't called Windows One:

Why Windows 10 wasn’t called Windows One: “It has been done before” (by Bill Gates)
A better question to ask now: Why did Microsoft call it Windows 10 specifically, and not something else? During the unveil event (video embedded above) Myerson gives us a few clues. Starting at around the 2:10 mark, he says: “We know, based on the product that’s coming, and just how different our approach will be overall, it wouldn’t be right to call it Windows 9.” He then talks about how Windows One would make sense with Xbox One, OneDrive, and OneNote, “but unfortunately Windows 1 has been done by the giants that came before us.” And so it seems the only other viable option was Windows 10.
Microsoft’s seemingly arbitrary naming convention of “Windows 10″ is an interesting one. It is clearly a strong version number — and it’s also a neat way of distancing it from Windows 8, which Microsoft really wants to bury in the living room couch cushions when no one is looking. In fact, this may even be the same trick that Microsoft used to make us forget about Vista: “Hey, with a name like Windows 7, it must be very different from Vista.”
Yosemite Safari

Apple’s OS X has been OS X for 14 years now — and shows no sign of being retired
What about any similarity to Apple’s Mac OS X? Apple did a similar thing: Its operating systems steadily incremented through System 1-7, then switched to Mac OS 8 and 9, and when it got to OS 10 (X) in 2001 it stopped. Given how Windows 10 is meant to be a single platform for just about every form factor, plus the massive weight and importance that Microsoft is lending to this release, we wouldn’t be surprised if it sticks around for a long time. We don’t think Microsoft is intentionally aping Apple with Windows 10, but the marketing department has to be aware of both the positive and negative repercussions of wanting to ride on Apple’s coattails.
So, there you have it: Windows 10 is called Windows 10 because Microsoft says so — even if “Windows” or “Windows X” would’ve been better. If you want to give Windows 10 a spin before its release, you can follow our guide and install the technical preview.