The figure determined by four lines, no three of which are concurrent, and their six points of intersection. Note that this figure is different from a complete quadrangle. A complete quadrilateral has three diagonals (compared to two for an ordinary quadrilateral). The midpoints of the diagonals of a complete quadrilateral are collinear on a line M.

complete quadrangle | Gauss-Bodenmiller theorem | midpoint | orthocenter | polar circle | quadrilateral