Dual arrangement of a planar point set
= Dual arrangement of a planar point set
Dualize a noncollinear finite point set in the <real projective plane> to projective lines. Their intersection vertices and consecutive line segments form an embedded graph. A vertex incident to $k$ lines has graph degree $2k$ and corresponds to a primal line containing $k$ points. <Ordinary lines> correspond to degree-four vertices. The faces are polygons with at least three sides.