Permanent mandibular lateral incisor is the second tool from the midline. Its tooth number in FDI system is 4, 2 and 3, 2. In universal system it is 26 and 23 and in Zygmunt De Palma system it is 2 with appropriate quadrant brackets. The function of mandibular lateral incisor is cutting. The tooth develops from four developmental lobes, mesial, labial, distal and lingual. The lingual lobe is less developed and forms the cingulum whereas the mesial, labial and distal lobe form the incisal ridge. Due to them, we see the three protuberances called mamillons. The age of eruption of mandibular lateral incisor is 7 to 8 years. The crown length is 9.5 mm and root length is 14 mm. The mesio distal diameter of the crown is 5.5 mm and at cervix it is 4 mm. The labiolingual dimension is 6.5 mm and at cervix it is 5.8 mm. The curvature of cervical line on mesial side is 3 mm and on distal side it is 2 mm. The morphology of the tooth is studied under 5 aspects. Let us start with the labial aspect. The outline form of the crown is trapezoidal with its longer side towards the incisal ridge and shorter side towards the cervical line. The incisal surface is straight but not horizontal. Its distal side is tilted in the cervical direction. This gives the crown a slight tilt on the root. Both incisal angles, mesoincisal angle and distoincisal angle are equally sharp. Coming to the contact areas, the mesial contact area by which the tooth contacts the central incisor is in the incisal third and the distal contact area by which the tooth contacts the canine is also in the incisal third but slightly cervically positioned. Both mesial outline and distal outline converge towards the cervix. The cervical line is convex with its convexity towards the root. The labial surface is smooth and convex. The root is smooth, conical and tapering and the apex usually has a distal tilt. Coming to the lingual aspect, as the lingual aspect is narrower than the labial aspect, all the features of the outline formed from the labial aspect remain same in the lingual aspect as well, although the crown surface is very different. Incisal ridge runs between the mesoincisal angle and distoincisal angle. Mesial marginal ridge is present on the mesial border and distal marginal ridge on the distal border. Cingulum is present in the cervical third. A concavity is seen in the middle of all these convexities called the lingual fossa. The root is smooth, conical and tapering and the apex has a distal tilt. Next the mesial aspect. The outline form of the crown is triangular in shape. The incisal ridge is not on the midline axis of the tooth but slightly lingual to it. The labial outline is convex with its maximum convexity in the cervical third. The lingual outline is convex in the cervical third, concave in the middle third and concave in the incisal third. The cervical line is convex and the convexity is towards the crown. Coming to the root, the root is smooth and conical in shape. A mesial developmental depression runs along the long axis of the root. Next, distal aspect. All the features from the mesial aspect are same in the distal aspect as well. Though, the distal developmental depression on the root is deeper than the mesial developmental depression. Finally, the incisal aspect. As with all permanent teeth, no part of the root is visible from the incisal aspect. The labiolingual diameter is more than the mesodistal diameter. The labial outline is smooth and convex and the lingual outline shows the lingual convergence. The incisal ridge is not straight. The distal half tilts lingually giving the crown a twist over the root. This is so that in a curving mandibular arch, the tooth has to maintain its mesial contact with central incisor and distal contact with canine.