By the way, those of you that measure thread the length of your arm might be interested to know if y ...
I think it's to do with an effort to standardise cloth lengths in the middle ages. At markets, cloth was measured from the seller's thumb to his nose and nobody would buy from a titchy man! Later on the yard was the length of the King's thumb to his nose. A yardstick was produced on this standard, and everyone got the same measure (may be apochryphal, but sounds reasonable)