What was the earliest equal temperament system used?
I believe it to be 19edo, used by Guillaume Costeley in Seigneur Dieu ta pitié, in 1558. Was any equal temperament used before then?
7edo is stated in many places to be used in Thai traditional music, but I can't find any evidence of that.
Don't count any piece that only uses octaves (since one-tone equal temperament is dumb).