Fig. 5. Occlusion of lower and upper molars during the chewing cycle (four phases) in superposition (A1–D1, left) and mesial aspect (A2–D2, right). In the superposition, the upper molar is drawn as if it were transparent. The phases of the chewing cycle were drawn after an experimental study with high precision epoxy casts. Arrows indicate direction of movement of lower molars. A. Initial contact. Cusp b of the lower molar contacts ridge A−C and moves dorso−medially towards the center of the upper molar (Phase 1). B. During this stage of the power stroke the volume between cusp b and cusps X−Y is compressed. A wear facet is produced at the buccal side of cusp b (Phase 2). C. Maximum occlusion. Cusp b fits snuggly into the deepest point of basin A−C−Y−X. Crest a−b fits into the deep valley of the distal margin of the upper molar between cusp Y and C. The inclined buccal side of cusp X meets the straight valley e−b−g (Phase 3). D. This stage of the power stroke reopens the jaws by a ventro−medial movement of the lower molar. Cusp X grinds along the straight valley e−b−g. This valley controls the movement of the lower molar and restricts it to a straight, transversal direction (Phase 4).