The tooth located in the permanent dentition of the mandible between the first and the third molar teeth. It has no primary tooth predecessor. The crown is similar in shape to the first permanent mandibular molar, although it usually only has four cusps (mesio-buccal, mesio-lingual, disto-buccal, and disto-lingual). The disto-buccal cusp is larger than in the first molar and located more distally. There are four grooves (fissures) separating the four cusps in a cruciate pattern. The buccal fissure usually extends onto the buccal surface. There are two roots which usually curve distally and may be fused for part of their length; they are normally shorter and weaker than those of the first molar. The pulp chamber has four pulp horns directed towards the four cusps. The root canal structure is very variable and is similar to the mandibular first molar. Calcification of the tooth begins at 2½–3 years after birth and the crown is normally complete by 7–8 years of age. The tooth erupts at about 11–13 years and the calcification of the root is complete at about 14–15 years.