The tooth located in the permanent dentition of the mandible between the posterior to the second molar tooth. It has no primary tooth predecessor and it is usually the smallest of the three lower molar teeth. The crown shows considerable variation and may present with either four or five cusps. The distal surface is markedly convex. The roots may be similar to those of the second molar but fusion of the roots sometimes leads to the formation of a simple cone-shaped root. There are usually only two root canals, a mesial and a distal. Calcification of the tooth begins at 8–10 years after birth and the crown is normally complete by 12–16 years of age. The tooth erupts at about 17–21 years and the calcification of the root is complete at about 18–25 years.