The fineness of the yarn has a variety of representation methods, such as number, metric count, inch count, denier, etc. (see count). The twist of the yarn is expressed in meters or meters per inch. Twist back to the direction of S and twist. In the range of a certain degree of twist, the strength of the yarn increases with increasing twist. The twist of the yarn and the twist of the strands are selected according to the use of the strands. Usually the single yarn and the opposite direction of the twist, that is, ZS or SZ. There is an optimum ratio between the singular yarn and the twist of the strands. In this range, the strength of the strands increases with the increase of the twist line, and the strength of the strands decreases after exceeding the critical value.