Phi Mu was not created by ADPi. Both ADPi and Phi Mu were founded at the same school however (so it's possible that they were "rivals"). Phi Mu was actually around first...but they were known as "The Philomethean Society" and ADPi was the "Adelphean Society" (someone feel free to correct me if I'm wrong) Anyway...ADPi was the first of the 2 organizations to become a Greek letter society, thereby making it the first "sorority" (I believe ADPi might be a fraternity...I know Phi Mu is) as we know it. Hope that helps somewhat.
