Described as either S or Z it indicates the direction of the twist of a yarn. When yarn is twisted towards the left (follows the diagonal direction of an S) it is an S twist, and when twisted to the right (following the diagonal direction of a Z) it is a Z twist. When two lengths are plied to make a thicker yarn, they will be twisted in the opposite direction, i.e. two S twist yarns will be plied by a Z twist together, or vice versa. The direction of this final twist determines whether it is a S or a Z twist yarn. See Alternating Twist.