Simply as an historical comment (the article doesn't go into it in any depth), the basis for the "X" system goes back to the days before the development of synthetic leader or tippet materials. The material referred to as "gut" or "catgut", which preceded the post-war synthetics was actually a part of the silk-spinning apparatus of the silkworm caterpillar. Stripped from the body of the caterpillar this short, flexible length of intestinal material could be stretched to an overall length of 18-20 inches (it is interesting that many leader formulas still call for tying custom leaders in 18-inch increments). After removal and stretching, this length of material was "fixed" by being dipped into an acid solution and then stripped through a series of diamond dies to reduce the diameter to a standard size; hence 1X, 2X, 3X, to represent the number of times the length of material was drawn through the serially smaller apertures.

The material was flexible so long as it remained moist so, in order to be stored in a useable condition, it was necessary to use the once-common leader box, the small, cylindrical aluminum box, where the 18-inch lengths of "gut" could be coiled and stored between pads of water-and-glycerine-saturated felt.