Windows versions

Windows, internally, have been versioned with numbers instead of codename like "Longhorn", "Vienna" etc. For example, Windows XP is internally versioned with "5.x", while Vista is basically Windows version 6. The next windows version (Windows 7) is trully windows version 7.
All are rooted since Windows 3.x Users mostly see product name (e.g, XP or Vista), but developers usually can see the versions (WinAPI doc from MSDN).  The numbers correspond to their major version and revision.  So expect a another big revision from Vista to Windows 7 (although users may not see or feel it, because the changes may be only internal to the system).

Here I try to list the versions:

Windows 3.0 -> winver 3
Windows 3.1 -> winver 3
Windows 95 -> winver 4
Windows NT -> winver 4
Windows 98 -> winver 4
Windows 2000 -> winver 5
Windows XP -> winver 5
Windows Vista -> winver 6
Windows 7 -> winver 7