The tooth that replaces the primary second molar located in the permanent dentition of the mandible between the first premolar tooth and the first molar. The crown is almost circular in outline and is larger than the crown of the first mandibular premolar but is subject to more variation. The two cusps are more equal in size and are united by a bucco-lingual ridge either side of which is a mesial and distal occlusal pit (fossa). The lingual cusp may be divided into two smaller cusps, of which the mesial is normally the larger. The mesial and distal marginal ridges are well formed; they bound the occlusal pits and connect the two cusps. Often the disto-buccal ridge may be absent so that a mesio-distal groove (fissure) connects the two pits. The mesial and distal surfaces are convex and slope towards the narrower lingual surface of the root. The lingual surface is wider than in the first premolar. There is a single root which, not uncommonly, curves distally towards the apex. It is oval in cross-section and flattened on the mesial and distal surfaces, often having vertical grooves. There is a single root canal with two pulp horns directed towards each cusp. Calcification of the tooth begins at about 2 years after birth and the crown is normally complete by 6–7 years of age. The tooth erupts at about 11–12 years and the calcification of the root is complete at about 13–14 years.