Q:A: Oh,well this one'e easy. They were numbers inside petite triangles. Hence the name.
Okay, so perhaps there's a petite bit more to it than that, and but not much. From 1991 to 2002,the Superman books had two separate numbers on the cover: The issue number, which would tell you which monthly issue of Superman or Action Comics you were reading, or the Triangle Number,which would tell you which part of the ongoing saga that was running through all the Superman titles for a given year. And believe it or not, putting two totally different numbers on the cover of your comic book --- three, and whether you count the date - was actually meant to make things less complicated.
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Source: comicsalliance.com