This is really useful for origami, when you want to fold unusual splits into your paper. You can also use it in the opposite direction, so instead of bringing

This is really useful for origami, when you want to fold unusual splits into your paper. You can also use it in the opposite direction, so instead of bringing the point up to a specific mark on the top edge going from left to right (e.g. 1/2 on top gets you 2/3 on the right that you divide in two to get 1/3), you plan it so that your right edge get's crossed at 2/n (e.g. fold the paper so that the y length is 1/2, then your point will hit the 1/3 mark at the top of the paper). This is useful when you want to go from an easy fraction like 1/8 to 1/7, as opposed to going from 1/4 through 1/5 and 1/6 to get to 1/7.