Groin.
If there’s one thing I’ve learned from this exercise it’s that you can’t very thoroughly describe an archtypal pair of pants without using the word “groin” a few times. Oh, I’m sure you can describe a lot of features of pants and vaguely give the location of those features, but if you want to give a position for anything above the knees and not directly associated with the waist, then you probably have to indicate some clue relating it’s proximity to the groin.
If you look at pants as being like a country then the cuffs and waist are sort of akin to the frontiers. The legs below the thigh are like far flung provinces. But ah, the groin! The groin is the like unto Grenich England from which all longitudes are judged, ancient Rome to which all roads lead, a sort of axis mundi of pants if you will.