By 1912, twelve states either selected delegates in primaries, used a preferential primary, or both. By 1920 there were 20 states with primaries, but some went back, and from 1936 to 1968, 12 states used them.
Thanks for that info. Made me check up on that.
One thing this election season has done, along with a whole lot more information available, was to push me to get much more educated about the primary process.