# Space of inscribed n-gons modulo projective transformations

Say P ~ Q (P and Q are «projectively equivalent») iff there is a projective transformation f such that f(P) = Q. Then ~ is an equivalence relation. I read that the space of inscribed n-gons modulo projective equivalence has dimension n-3. Why is this? Also, are there any related results?

-
What is an "inscribed n-gon"? Inscribed in a circle? –  Joseph O'Rourke Aug 5 '10 at 22:43
Sorry, I mean inscribed in a conic. –  Adeel Aug 5 '10 at 23:49
Conic or circle doesn't make a difference (projectively), but it does matter what you mean by an N-gon. Here it means "an ordered set of N distinct points". Other meanings change the space of polygons, though its dimension is (N-3) under any interpretation. –  T.. Aug 6 '10 at 0:21