If 4 points in a plane are joined 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 if 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 1.4 A (Left), the quadrangle is PQRS, its sides are PS, QS, RS, QR, RP, PQ, and its diagonal points are A, B, C.