The X-system was of course designed by the proud people who still resist going metric. English or American. They all seem to have a strong dislike towards anything concerning the number ten. Twelve seems to be a number they are fond of, but they really can't stick to it consistently. Enter the X. And the 11 rule.
"Back in the olden days when cat gut was used as leader/tippet, it had to do with the number of times the gut was pulled through an "extrusion" tool to taper it down. The way it works is a piece of cat gut of 0.011 inches was used as a starting point. It was assigned the "0X" designation. Pulling it through the tool once would shave it down by 0.001 inches, making it 0.010" and a designation of "1X" because it was pulled through 1 time, or 1x. So, any combination of inches and X's that equals eleven is the "11 Rule". Another example, a 5x leader was pulled through 5 times, subtract this from 11, and you end up with a leader with a 0.006" diameter."Quote from the forum at www.fluefiskenett.com. Original author not mentioned.
Aren't you all glad that it was the Arabs and the Greeks and not these guys who invented mathematics?
Courtesy of Vegard V.
The X system is about two hundred and fifty years old and is based on an international system used for sizing the wire used to make watch parts. When gut first began to be drawn, by sharpening the end of the strand and pulling it through the bored centre of a jewel, the manufacturers found they needed a common measurement system and after casting around without much success, they settled on the watchmakers' old favourite.
The X system was perfectly adequate for gut, but in the metric age it is gradually falling into disuse, because the very wide variation in diameter of nylon leaders means that measurements frequently fall outside the range of the old scale.
This quote is attributed to Dr. Andrew Herd and supplied by Ben Spinks