1) The facial and lingual aspects of all tooth crowns can be outlined using three basic geometric shapes: triangles, trapezoids, and rhombuses. 2) The proximal outlines of anterior teeth are triangular, with the base toward the cervix and apex toward the incisal ridge. 3) The proximal outlines of maxillary posterior teeth are trapezoidal, with the largest side toward the cervix and shortest toward the occlusal surface. 4) The proximal outlines of mandibular posterior teeth are rhomboidal.