Twist coefficient：
Twist can not be used to compare the twisting degree of different roving yarns, because at the same twist, roving yarns have a more inclined fiber than fine yarns. In practical production, twist coefficient is often used to express the twist degree of yarn. Twist coefficient is the relative value of the twist degree of yarn expressed by the linear density, which can be used to compare the twist degree of different roving yarns. Twist coefficient can be calculated according to twist and yarn density.
Twist direction：
Twist direction refers to the inclined direction of the single yarn in the single yarn or the single yarn in the ply yarn after twisting. It is divided into Z twist and S twist. After twisting, the twist direction of yarn tends to the upper left corner from the lower right corner, and the tilt direction is consistent with the middle of “S”, which is called S twist or smoothly twist; the twist direction of yarn tends to the upper right corner from the lower left corner, and the tilt direction is consistent with the middle of “Z”, which is called Z twist or backhand twist. Z twist is often used in single yarn and S twist is used in ply yarn.
Twist shringage：
After twisting, the yarn length is shortened due to the inclination of the fibers, resulting in twist shringage. The size of twist shringage is usually expressed by twist shringage ratio, that is, the difference of yarn length before and after twisting is the percentage of the length before twisting.
The twist shringage ratio directly affects the yarn density and twist. It must be considered in the process design of spinning and twisting. The twist shringage ratio of cotton yarn is generally 2%~3%. Besides the twist coefficient, the twist shringage ratio is also related to the spinning tension, the temperature and humidity in the workshop, and the yarn thickness.