A furlong measures 1/8 of a mile, but where did that measurement originate? The word dates back to the 9th century, and is “a furrow long”—the distance eight oxen in a plow team can work without rest.
Other medieval measurements you may not have heard of:
Ell – A measurement of length, from the tip of the middle finger to the elbow.
Gill – a small liquid measure, ranging from a modern shot glass to 1/4 of a pint.
Nail – (1) A measure of weight for wool and beef, equivalent to about 8 pounds. (2) A measure of land. (3) A measure of length for cloth, approximately 1/16th of a yard.
“The precise origin of this sense is not clear. The use of the nail in early examples suggests that one sixteenth from the end of the yard-stick may have been marked by a nail.” (OED)
Span – The distance from the tip of the thumb to the tip of the little finger, or sometimes to the tip of the forefinger, when the hand is fully extended (about 9 inches).
Mark – A measure of weight, usually for gold and silver. Though its actual weight could vary a lot, it was considered to be about 8 oz.