On a given line, if two ranges I and II are defined, then we can assign a point on the line to either range.[br]Consider a projection I → II, where at the same time[br]在给定的直线上，如果定义了两个范围I和II，那么我们可以将直线上的一个点赋值给这两个范围。[br]考虑投影I→II，同时[br][color=#ff0000] A → A'[br] A' → A[/color][br]Then {A, A'} are referred to as a [i]conjugate pair.[/i][br]那么{A, A'}就称为共轭对。[br]If, in a projection two pairs of corresponding points are conjugate, then every pair is conjugate (Desargues). The projection is determined, but what is it? Given conjugate pairs {A, A'}, {B, B'}, and an arbitrary point C on the same line, where is C'?[br]如果在一个投影中，两对对应点是共轭的，那么每对点都是共轭的(desargue)。投影是确定的，但它是什么呢?给定共轭对{A, A'}， {B, B'}和同一直线上的任意点C，请问C'在哪?