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Coxeter- Figure 1.4A (left)

If 4 points in a plane are joined in pairs by 6 distinct lines, they are called the vertices of a complete quadrangle, and the lines are its 6 sides. Two sides are said to be opposite is their common point is not a vertex. The common point of two opposite sides is called a diagonal point. There are 3 diagonal points. In Figure 1.4A, the quadrangle is PQRS, its sides are PS, QS, RS QR, RP, PQ, and its diagonal points are A, B, C.