I'm doing reading course on Projective Geometry. I was presented this question (in the textbook, not homework): In [itex]P_2 R[/itex], let A, B, and C be points on a line L and let A', B', and C' be points on a line L'. Prove there exists points [itex]S_1[/itex], [itex]S_2[/itex] and [itex]S_3[/itex], and lines [itex]l_1[/itex] and [itex]l_2[/itex] such that projection from L to [itex]l_1[/itex] with center [itex]S_1[/itex], ... And it continues. The remainder is not important. What do they mean by center [itex]S_1[/itex]? Thank you.