The tooth located in the permanent dentition of the mandible between the second premolar tooth and the second molar. It has no primary tooth predecessor and erupts immediately posterior to the second primary molar. The crown is longer mesio-distally than bucco-lingually and longer buccally than lingually. There are four large cusps (mesio-buccal, mesio-lingual, disto-buccal, and disto-lingual) and one small distal cusp. The four large cusps are separated by a cruciate pattern of grooves (fissures) with a central pit (fossa). The lingual fissure may extend onto the lingual surface and the buccal fissure may extend onto the buccal surface and may end up in a small pit which is often a sight of a carious lesion. The distal fissure divides, one arm extending over the buccal surface and the other dividing the distal and disto-lingual cusps and possibly extending onto the distal surface. The mesial cusps are connected by a mesial marginal ridge, and the distal and disto-lingual cusps are connected by a distal marginal ridge. The four walls of the crown are all convex, particularly the buccal. There are two roots, one mesial and one distal united by a common undivided part below the crown. Both roots are broad and flattened on the mesial and distal surfaces. The mesial root is inclined distally and has a marked vertical groove on its distal surface. The smaller distal root has a narrower vertical groove on its mesial surface and may show little distal curvature. The pulp chamber has four pulp horns directed towards the four large cusps. There are normally two root canals in the mesial root and one in the distal root but this is subject to considerable variation with the distal root sometimes having two root canals. Calcification of the tooth begins at or shortly before birth and the crown is normally complete by 2½–3 years of age. The tooth erupts at about 6–7 years and the calcification of the root is complete at about 9–10 years.