The designation by Edward H. Angle of the types of malocclusion based on the relative positions of the first molar in the two arches when in occlusion: Class I — normal anteroposterior relationship but with crowding and rotated teeth. Class II — the lower arch is distal to the upper arch on one or both sides; the lower first molar is distal to the upper first molar. Class III — the lower arch is anterior to the upper arch on one or both sides; the lower first molar is anterior to the upper first molar.