One of two teeth located between the mandibular central incisors and the canines in the permanent dentition. They replace the mandibular primary lateral incisor teeth. It is similar to the lower central incisor but is wider mesio-distally and therefore more fan-shaped. The lingual surface usually shows more definite mesial and distal marginal ridges. On eruption, the incisal edge has three small tubercles (mamelons), which wear away to give a flat surface. 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 7–8 years and the calcification of the root is complete at about 10 years.