Black triangle (BT): During the orthodontic process, while the roots rotate and move, the gingival fibers between the adjacent teeth are pulled, and the gingival papillae between the incisors also recede to different degrees due to the force, and the gingival papillae originally attached to the crowns are lost, resulting in a triangular space below the contact points of the adjacent teeth, which is clinically called the black triangle. The black triangle not only poses functional problems, but also affects pronunciation and aesthetics. Factors associated with the formation of the black triangle: 1. Age Studies have shown that the incidence of the black triangle in people over 20 years of age is 67%, while in people under 20 years of age it is 18%. There is a correlation between age and the black triangle, with the height of the gingival papilla decreasing by an average of 0.012 mm per year as age increases. 2. Crown shape Studies have found that cuspate crowns are the most likely to develop the black triangle, while square crowns are the least likely to cause the black triangle. For patients with thin alveolar bone in the anterior region, attention should be paid to the way the anterior teeth are moved to avoid any factors that cause alveolar bone resorption and to prevent the formation of the black triangle as much as possible. The distance between the top of the alveolar crest of two teeth and the adjacent point of two teeth is an important factor in determining whether the black triangle will occur. 4.Inter-root separation angle The inter-root separation angle is the angle formed by the long axis of the adjacent teeth. It includes the inter-root separation angle of natural teeth and the inter-root separation angle formed by orthodontic treatment. The normal inter-incisor inter-root separation angle is 3.65°. For every 1° increase in this angle, the incidence of black triangle increases by 14%-21%. 5.Crowding Severely crowded teeth are always cited as a factor for the appearance of the black triangle after orthodontic treatment because the interdental gingival papillae are squeezed or stretched during the alignment process. No statistically significant difference was found between gingivitis and the formation of the black triangle, but the incidence of the black triangle tended to increase in patients with moderate to severe crowding with gingivitis and in patients with significant overlap of maxillary central incisors. Methods of eliminating the black triangle: 1, adjacent surface de-enamelization. 2.Do composite resin restoration or porcelain veneer. 3.Change the angle of the tooth root to change the position of the contact area. 4.Surgery – gingival papilloplasty.