The direction of twist in yarns and other textile strands is indicated by the capital letters S and Z. Yarn has S-twist if when it is held vertically, the spirals around its central axis slope in the same direction as the middle portion of the letter S, and Z-twist if they slope in the same direction as the middle portion of the letter Z. When two or more yarns, either single or plied, are twisted together, the letters S and Z are used in a similar manner to indicate the direction of the last twist inserted.