The tooth located in the permanent dentition of the maxilla between the second premolar tooth and the second molar. The occlusal surface of the crown is diamond-shaped with the longest diagonal extending from the mesio-buccal corner to the disto-lingual corner. It has four cusps (mesio-buccal, mesio-palatal, disto-buccal, and disto-palatal) separated by an H-shaped configuration of grooves (fissures). Mesial and distal marginal ridges connect the mesial and distal cusps respectively. An oblique ridge joins the mesio-palatal and disto-buccal cusps to divide the occlusal surface into a larger mesial and smaller distal area both containing pits (fossae). A buccal fissure from the mesial pit extends onto the buccal surface. A fissure runs from the small distal pit parallel to the oblique ridge and turns on to the palatal surface of the crown. A fifth cusp, or small elevation (the tubercle of Carabelli), is sometimes found on the palatal surface about midway between the apex and the cervical margin. The buccal, palatal, and distal surfaces of the crown are convex, whereas the mesial surface is almost flat. There are three roots (mesio-buccal, disto-buccal, and palatal) which remain united towards the crown. The buccal roots are flattened on their mesial and distal surfaces and are shorter than the much longer and straighter palatal root which diverges widely from the other two roots. The pulp chamber has four horns extending into the base of each of the four cusps. There are normally three root canals but in many cases there may be four or more. Calcification of the tooth begins at or shortly after 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.