Mandibular first molar is the earliest eruption of permanent teeth, characterized by buccal slightly trapezoidal, lingual trapezoidal, neighboring surface like a quadrilateral. 1. The buccal surface is slightly trapezoidal: because the occlusal margin of the teeth is longer than the cervical margin, the proximal and distal mid-diameter is larger than the cervical margin of the teeth, the proximal mid-margin is straight and the distal mid-margin is protruding. The occlusal margin is half of the proximal, distal and distal buccal cusps, separated by the buccal sulcus and distal buccal sulcus, respectively, and the buccal sulcus ends in a point gap. 2. The lingual surface is trapezoidal: because the tooth is smaller and slightly more rounded than the buccal surface. The occlusal margins are visible proximally and distally to the mesial tip of the tongue, the lingual groove is picked up from the tip of the tongue, and the lingual axial ridge is not obvious. The highest point is in the 1/3 of the tongue. 3. The neighboring surfaces appear to be quadrilateral: because the crown of the tooth is tilted toward the lingual side, the buccal cusp is lower than the lingual cusp, the distal mesial surface is smaller than the proximal mesial surface, the mesiobuccal cervical angle and mesiolingual angle are acute. The proximal and distal contact areas are close to 1/3 of the jaw side. If you experience discomfort such as toothache, you should go to the hospital immediately for treatment.