Shown below some tai chi rulers with long weapons that feature crescent hand guards. I conjecture that there should be a relationship between measurements of the student (specifically, the distance across the chest from Large Intestine 15 on the left arm to Large Intestine 15 on the right arm as well as the distance from Pericardium 9 at the tip of the middle finger to Lung 10 at the base of the thumb) and the length of the ruler and diameter of the pommels of the ruler.