Hello, I cannot remember what the theorem is in which the following happens: Given two lines l and m which intersect each other, let H be the point of intersection. Let A and B be points on the line l such that AHB are colinear. And let C and D be points on the line m such that CHD are colinear. Now what is the theorem/lemma/corollary which states that when two such lines intersect in such a way that angle AHC = angle BHD and angle AHD = angle BHC ? I need to quote it for a proof that I am doing. I can't remember for my life. And I can't seem to find it in my notes/text. It's not a big problem, I would just like to quote it properly. Any help is appreciated. Thankyou.