Point-pair separation

In a cyclic order, such as the real projective line, two pairs of points separate each other when they occur alternately in the order. Thus the ordering a b c d of four points has (a,c) and (b,d) as separating pairs. This point-pair separation is an invariant of projectivities of the line.

The concept was described by G. B. Halsted at the outset of his Synthetic Projective Geometry: "With regard to a pair of different points of those on a straight, all remaining fall into two classes, such that every point belongs to one and only one. If two points belong to different classes with regard to a pair of points, then also the latter two belong to different classes with regard to the  first two. Two such point pairs are said to 'separate each other.' Four different points on a straight can always be partitioned in one and only one way into pairs separating each other."

Given any pair of points on a projective line, they separate a third point from its harmonic conjugate.

A pair of lines in a pencil separates another pair when a transversal crosses the pairs in separated points.