In mandibular first molar the outline form of the crown from the occlusal aspect is hexagonal. Its mesio-distal dimension is more than the bucco-lingual dimension, which is opposite to that in the maxillary first molar. The mesio-distal dimension is more in the buccal half than the lingual half and the bucco-lingual dimension is more in the mesial half than the distal half. From the occlusal aspect, all 5 cusps are seen. The mesio-buccal cusp is the largest, followed by the mesio-lingual cusp and disto-lingual cusp which are almost of equal size. The distobuccal cusp is smaller and the smallest is the distal cusp. The 2 buccal cusps and two lingual cusps are considered the primary cusps. Whereas, the distal cusp is the secondary cusp. There are numerous ridges in the mandibular first molar, starting with the cuspal ridges. Each cusp has 2 cuspal ridges, 1 mesial cuspal ridge and 1 distal cuspal ridge. 5 triangular ridges, 1 for each cusp, is seen running from the cusp tip towards the center. 2 marginal ridges are present, mesial marginal ridge present at the mesial outline and distal marginal ridge present at the distal outline. There is 1 major fossa and 2 minor fossa. The major fossa is the central fossa, which is roughly circular in shape and is present in the center of the occlusal surface. The 2 minor fossa are the mesial triangular fossa and distal triangular fossa, which are adjacent to the respective marginal ridges. There are 3 pits, the central pit is seen at the bottom of the central fossa. It is the junction of central, lingual and mesio-buccal developmental grooves. Mesial pit is present in the mesial triangular fossa and distal pit in the distal triangular fossa. There are 4 developmental grooves, 1 central developmental groove which runs mesio-distally on the occlusal surface. It starts from the mesial pit in the mesial triangular fossa, crosses the central pit and ends in the distal pit in the distal triangular fossa. The lingual developmental groove originates from the central pit, runs lingually between the lingual cusps onto the lingual surface. The mesio-buccal developmental groove also originates from the central pit and runs buccally between the mesio-buccal cusp and disto-buccal cusp, onto the buccal surface. The disto-buccal developmental groove originates from the central developmental groove slightly distal to the central pit, runs buccally between the disto-buccal cusp and distal cusp, onto the buccal surface. Apart from these, numerous supplemental grooves are also seen on the occlusal surface. Some clinical considerations. Mandibular first molar along with maxillary first molar is used to classify malocclusion in teeth according to Angle's classification. Due to its two strong roots, mandibular first molar is used as an anchorage in orthodontic treatment. Mandibular first molar is the most common tooth to have caries, this is because of two reasons, it is the first permanent tooth to erupt in the oral cavity and so is exposed to oral bacteria the longest, second, it has numerous grooves and pits which lead to lodgment of food. The mesial root usually has 2 root canals, mesio-buccal and mesio-lingual, this is important for endodontic procedures.