Coxeter-Figure 1.4 A (right)

If 4 lines in a plane are joined by 6 distinct points, they are called the vertices of a complete quadrilateral, and the points are its 6 vertices. Two vertices are said to be opposite if their join is not a side. The join of two opposite vertices is called a diagonal line. There are 3 diagonal lines. in Figure 1.4 A (Right), the quadrangle is pqrs, its vertices are p.s, q.s, r.s, q.r, r.p, p.q, and its diagonal lines are a, b, c.