Suppose the polygon is ABCDEFGHIJKLMNOPQ Then each choice of 2 letters represents either a side or a diagonal. So there are C(17,2) sides and diagonals altogether. There are 17 sides, AB, BC, CD, ..., PQ so the number of diagonals is 17 less than C(17,2). Answer: C(17,2) - 17 = 136 - 17 = 119 diagonals. Edwin