The Windows version numbers are not used often but really do help group Windows into distinct "early vs late" product cycle tiers. They didn't really skip straight to 10, they just named 9 8.1 for reasons.

Windows 5.0-5.2 is Win 2000, Win XP, Win XP64.

Windows 6.0-6.3 is Vista, 7, 8, 8.1.