The version of Windows following 8.1 will be Windows 10, not Windows 9. Apparently this is because Microsoft knows that a lot of software naively looks at the first digit of the version number, concluding that it must be Windows 95 or Windows 98 if it starts with 9.
Many think this is stupid. They say that Microsoft should call the next version Windows 9, and if somebody’s dumb code breaks, it’s their own fault.
People who think that way aren’t billionaires. Microsoft got where it is, in part, because they have enough business savvy to take responsibility for problems that are not their fault but that would be perceived as being their fault.
Conversely MS know they can deliver a crappy web browser knowing it’s the web site that will be blamed.
The internal version number of Windows 8.1 is 6.3 .
From the days when I wrote code against the Win32 API (i.e. the one used in Windows 95 and 98), the easiest way to get an idea which version of Windows you are running is the GetVersion or GetVersionEx calls as described on e.g. CodeGuru. This doesn’t return a String “Windows 95”, but an actual version number akin to 4.0 (Windows 95), 4.10 (Windows 98), 4.90 (Windows Me), …, 6.1 (Windows 7) until 6.4 (Windows 10) but in a data structure.
I have a hard time believing that Microsoft would base its decision on a shoddy API that returns a version in a String: it’s shouldn’t be used by any reliable piece of software. Such software could probably only work in a compatibility layer, and even there Microsoft doesn’t have a perfect track record.
Another interpretation might be that in Japan, the number 9 means suffering (or is sometimes pronounced the same). This doesn’t seem that ridiculous, given that many buildings don’t have a “13th” floor in some western countries to avoid any superstitions.
Another explanation: it puts Windows 10 on par with OS X 10.10 (Yosemite). People still think in terms of bigger (numbers) are better, even for unrelated things. In browsers we saw a similar version number race between IE, Chrome and Firefox (luckily, those version numbers are almost invisible nowadays).
It could also be a great marketing instrument to generate posts speculating about skipping version 9, hence bringing the new version into the news more often (without costing a single dime). In a way, it also creates the image that from Windows 8 to 10, there have to be more and better changes than just going to version 9.
I remember Joel Spolsky writing about Microsoft taking responsibility for others’ mistakes back in 2004. Search on that page for “SimCity” to hear one interesting story.