Materials & Techniques
Direction of Twist
The direction of twist (rope-making term, lay) is designated S or Z according to whether the trend of the spiral of the spun or twisted element conforms, when held in a vertical position, to slant of the central portion of the letter S or the letter Z; and, in print, the appropriate slanting lines ( \ and / ) are sometimes used in place of the letters to indicate the direction of a twist. Lack of twist is usually designated zero twist, although there seems to be merit in the suggestion that the use of the letter I would be more consistent with the use of S and Z.
Irene Emery, The Primary Structures of Fabrics (Washington D.C.: The Textile Museum, 2009): 11.