Why skip a version number? Microsoft is positioning Windows 10 as such a major leap over what has come before that it deserves a two-number jump. The OS was originally code-named Threshold, before the surprise announcement of the official Windows 10 name.
"We want all these Windows 7 users to have the sentiment that yesterday they were driving a first-generation Prius," said Microsoft's Joe Belfiore, corporate VP of operating systems, "and now with Windows 10 it's like we got them a Tesla."
While Windows 8 aggressively courted touchscreens, some would say to the detriment of everyone else, Windows 10 aims to work equally well with a mouse-and-keyboard desktop as with a touchscreen tablet.
Windows 8.1 attempted to pull back some of the design excesses of Windows 8, including the return of something close to the traditional Start menu. In Windows 10, the Start menu is back from the start, with many of its classic functions restored.
Windows 10 will be able to adjust your user view, and the features it offers, by detecting what type of device or software you're using, for example, switching to a more touch-friendly view when you detach your keyboard.
The mobile phone version will also be called Windows 10, but its design and functionality have not been revealed yet. Microsoft is launching a preview program to let developers and others try the very early OS and influence its development. The final version of Windows 10 is expected in late 2015.