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