If you REALLY want to get technical about the numbers, After NT 4, NT 5 was sent out for beta testing, but it was too late in the cycle, and they pulled it. Windows 2000(was completely different then NT5) (should be Windows 6) and then ME (Windows 7 -Technical), then XP (Windows 8, but called 5.1), then Vista (Windows 9) and Finally "Windows 7" should be Windows 10 in my book, and thats not including all the "server" packages which are technically different.