# Phase Crossover Frequency at 0 deg or -180 deg

I've learned in school to calculate gain margin by first finding the phase crossover frequency at -180 deg, but sometimes I see people calculate gain margin by finding the phase crossover frequency at 0 deg which doesn't make sense to me. I'm confused as to why that method is used? I can't find any explanations online so any help is appreciated.

• it must be different methods as one includes the 180 deg inversion – Tony Stewart Sunnyskyguy EE75 Jan 3 '20 at 3:38