There are two kinds of right-angle triangles, which are common right-angle triangles and isosceles right-angle triangles. In a right triangle, the two sides adjacent to the right angle are called right angles, and the side opposite the right angle is called hypotenuse. The right side of a right triangle is also called a "chord". If two right-angle sides are not the same length, the shorter side is called "hook" and the longer side is called "strand".

Rulers used in different situations have different requirements and standards. If the value is more accurate, it requires higher technology and must be produced by machine. In addition to the requirement of accurate value, the second key is the quality of the engraved value. Generally, a protective film will be applied after the engraved value. The third point is the selection of materials, which are now divided into plastic, metal and wooden rulers, Plastic rulers are mainly used for stationery by ordinary students, metal rulers are more used in industry, and wooden rulers are mainly used for teaching.