If you look at the angles around the fold, you find that if you number the angles in a circle, all the even-numbered angles add up to a straight line, all the odd-numbered angles add up to a straight line.
Ako gledate kuteve oko preklopa, nalazite da ako numerirate kuteve u krug, svi parni kutevi zbrojeni  daju ravnu liniju, svi neparni kutevi zbrojeni  daju ravnu liniju.