A complete quadrangle consists of four projective points in the projective plane, no three collinear, and all six joining projective lines. Its three diagonal points are the intersections of opposite joining lines. Over a field of characteristic different from two, the line joining two diagonal points cuts either side through the third diagonal point in a pair of harmonic conjugate points. In a projective frame this follows by using the coordinate points and and computing the intersections.
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A **complete quadrangle** is a geometric configuration consisting of four points (vertices) that are not all on the same line, along with the six lines that connect each pair of points. More specifically, these four points form a set of lines, and every pair of distinct points is connected by a line segment.