The tooth located in the primary dentition of the mandible distal to the primary canine tooth. It is succeeded by the maxillary first premolar. The occlusal surface of the crown is irregularly quadrilateral with the buccal side longer than the lingual and an obtuse mesio-palatal angle. The occlusal surface is divided into buccal and palatal parts by a deep groove running mesio-distally terminating in poorly defined mesial and distal marginal ridges. The buccal surface is markedly bulged and may be accentuated in the mesial half to create a tubercle (the tubercle of Zuckerhandl). It has three roots, mesio-buccal, disto-buccal, and palatal, which are markedly divergent to make room for the tooth germ of the maxillary first premolar. The palatal root is the largest and longest and is almost circular in cross-section. The other two roots are flattened in a mesio-distal direction. The palatal and disto-buccal roots are sometimes fused. Calcification of the tooth begins at about 5 months of foetal life. The tooth erupts at about 14 months after birth and the calcification of the root is complete at about 2½ years.