The origin cannot be traced, but the name has its origin: 1. Obtuse angle: "A dull knife cuts flesh - the more it cuts, the more painful it becomes." The ancients found that the shape of this kind of angle is very similar to that of a "dull knife", so it is called "Obtuse angle". 2. Right angle: In ancient times, when building a house, ink lines must be drawn, and the magic fairy requires it to be perpendicular to the ground. In life, such an angle is called a "vertical angle", or "right angle" for short. 3. Acute angle: "Unbearable sharpness" "The most important thing for ancient armies to attack was their sharpness, and this kind of angle is similar to the sharpness of the army's formation, so it is called "acute angle".