When Twist? is inserted into a CF yarn, starting at its free end, the filaments adopt a helical shape (or spiral) which may be a clockwise or counter clockwise spiral, in accordance with the direction of rotation of the twisting device. With the clockwise spiral, the inclination of the helix mirrors the inclination in the letter Z, and the Twist? is called Z-Twist?. Similarly with counter clockwise the Twist? is referred to as S-Twist?; see. Fig.3. Importantly, after the action of twisting the inserted Twist? remains in the yarn[1].