I recently used a yarn swift, the sort that sits flat on a table top. It looks like a propeller with pegs sticking up to hold the yarn, and the pegs go in slots that correspond to circumferences of 72 inches, 60 inches, and so on.
I use a two-yard circumference niddy noddy to create my skeins, and every time I've given the yardage of a skein here on the blog I have counted the number of threads, divided by two, and called the result the number of yards.
After using the swift, I realized I should really make up a unit of measurement to attach to the results of my math, because whatever I have been getting cannot be derived from a full two-yard circumference. Not according to the position of the pegs on the swift. I knew that yarn wrapped around a niddy noddy is under tension and that a wool skein contracts somewhat when removed, but I never dreamed my skeins were quite so far gone. I don't even think they measure a smoot.