There are a variety of methods to express yarn count, such as number and metric count and so on. The twist of a yarn is expressed by the number of turns per metre or inch. The twist direction is divided into S twist and Z twist. Within a certain twist range, the strength of the yarn increases with the twist. The twist of the single yarn and the twist of the strands are chosen according to the use of the strands. In general, single yarns and strands are twisted in the opposite directions, like, ZS or SZ. There is an optimum ratio of twist between single yarn and strand. In this range, the strength of the strands increases with the twist of the strands, and the strength of the strands decreases when the critical value exceeds the critical value.