The tooth located in the permanent dentition of the maxilla distal to the second molar tooth. It is similar in form but smaller than the second molar tooth and is the most variable in the human dentition. Although it may have four cusps, it is most commonly present as a tricuspid tooth with the disto-palatal cusp and distal fossa being missing. A further reduction in size of the crown may present as a peg-shaped tooth with a small cone-shaped crown. Sometimes there may be one or more accessory cusps. The distal surface of the crown is usually more convex than that of the first and second molars. There may be up to three roots but, more often, the roots are fused along most or all of their length and frequently show irregular curvature. There are many variations in the size and shape of the pulp chamber and the number of root canals. Calcification of the tooth begins at 7–9 years after birth and the crown is normally complete by 12–16 years of age. The tooth erupts at about 17–21 years, but this is very variable and sometimes it may fail to erupt at all. The calcification of the root is complete at about 18–25+ years.