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Windows 3.1x was the first popular Windows system. Released as an expansion from Windows 3, it was a stepping-stone in moving from the DOS era to the Windows 9X Kernel. Windows 3.1 was the first edition of Windows to support Multiprogramming. It also introduced TrueType, and the Windows Registry.
Windows 3.1 also removes CGA Graphics, and Real Mode support which were included in Windows 3. Windows for Workgroups shares lineage with Windows 3.1x.
Windows 3.1 was the last 16-bit Windows Operating System, being replaced with the 16/32-bit hybrid Windows 95 in 1995.
21st Century Use
Windows 3.1 can be run in DOSBox fairly well. Some things crash, but plenty works. An Installation guide is available here.
With Win3.1 running you can then checkout some classic stuff from the 386 era: