Source -With Windows 7 and Windows Server R2 release Client and Server are in sync again, this was the case back in NT 3.1 in 1994, NT 3.51 in 1995, NT4 in 1996 and Windows 2000 in 2000. With Codename Whistler Windows XP (NT5.1) and .NET server (later named 2003, NT5.2) release went out of sync. Windows Vista (NT6.0) and Windows Server 2008 (NT6.0) the kernel got back in sync (not the release).
Windows 7 which is actually NT 6.1, so codename Windows 8 is NT 7 .
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