Used Power Pole Anchor Why Do Some Power Lines Use 3 Wires And Others Only 2?

Why do some power lines use 3 wires and others only 2? - used power pole anchor

I live in rural Montana and centers of power have only 2 lines. I noticed 3 in other areas near large cities. Perhaps the line 3 is a return of land to the utility. Perhaps the type of line 2 is more than "land" in rural areas instead of a return line for the country, but I'm not sure.