.the three rules

If we add a mountain fold at this vertex,there are three valleys and two mountains.If it’s a valley, there are four valleys and one mountain.Either way, the model doesn't fall flat.The third rule is that if we number all the angles at an interior vertex moving clockwise or counterclockwise,the even-numbered angles must add up to 180 degrees,as must the odd-numbered angles.Looking closer at the folds, we can see why.