One of two teeth which are the smallest in the permanent dentition located between the mandibular lateral incisors either side of the midline. They replace the mandibular primary central incisor teeth. It has a convex labial surface with a flattened area in the incisal half. The lingual surface is concave, except above the cervical margin where it is convex. Unlike the upper central incisors, there is no well-defined cingulum and no definite marginal ridges on the lingual surface. The mesial and distal surfaces of the crown are triangular in shape, to give a wedge-shaped appearance, and are almost at right angles to the incisal edge. On eruption, the incisal edge has three small tubercles (mamelons), which wear away to give a flat surface. It has a single root, oval in cross-section and flattened on the mesial and distal surfaces. The apical region may show some degree of bifurcation. The pulp chamber has two mesial and distal pulp horns directed towards the mesial and distal angles of the incisal edge. The root canal is subject to variation and may divide to give buccal and lingual canals. Calcification of the tooth begins at about 3–4 months after birth and the crown is normally complete by 4–5 years of age. The tooth erupts at about 6–7 years and the calcification of the root is complete at about 9–10 years.