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".

The most taboo thing about rulers is the scratching and blurring of the engraved value, so rulers become the most protected articles in stationery. The use of rulers should be correct. For example, it is better not to use plastic rulers when delimiting materials, which will easily be scratched. Many people will make a mistake, that is, they will draw the line very fast, and then they will run along the ruler because of the slippery characteristics of the ruler, which will make the value, so the speed is moderate.