Let's start with the mile. The mile goes back to the Romans, who said that a "mile" was one thousand paces (one thousand is mille in Latin). A pace was left-foot-right-foot, so we would say that each pace was a bit over five feet if using our mile. They said a mile was 5000 feet. How long was a foot? As long as a person's foot! (that was easy). But whose foot? One emperor suggested that his foot was the standard, but there was nothing like the modern standard. We do know, based on surviving Roman mile posts, that their mile was somewhat shorter than ours.
(And don't get me started on nautical miles, which even today are slightly longer than regular miles. Knots today are nautical miles an hour. There's a reason most of the planet went metric.)
In the Middle Ages, distances were given in miles, but unless old Roman mile stones were still in use, their exact length was subject to debate. In England, more important were furlongs, which were supposed to be 1/8 of a mile. They were "as long as a furrow" which is where the name comes from—furrows were the distance one would plow before turning the plow and going back the other way. English Parliament came up with absolute standards for the length of a foot, a rod, a furlong, and a mile in 1593, and we've followed them ever since. (Furlongs are still used in horse racing.)
As for shorter units, a yard was the standard measurement for cloth. This was supposed to be from a man's nose out to the end of his outstretched hand. Because most medieval men were less than six feet tall, their yard was smaller than our yard. It also varied considerably. During the Champagne trade fairs, cloth would have to be unwound from the bolt and measured and measured again using each town's own idea of a yard. In England, where there was concerted effort in the Middle Ages for everyone to use the same measurements, the yard was supposedly based on a certain king's reach.
Then there were pounds, another unit gotten from the Romans. As in the case of yards, if one were selling goods at a trade fair one's goods would have to be weighed and reweighed in every town, to match their weights. French trading centers would have been mortified to have to use someone else's units of measure. (Gold is still measured in troy-weight, the measure used in Troyes.)
Medieval documents often gave lengths in perches when specifying boundaries. The Roman pertica had been ten Roman feet long. By the early modern period in France, the perche was supposed to be eighteen feet (royal feet). Medieval perches were, it seems, somewhere in between. The modern rod is the modern equivalent of the perche and is now defined as 16 1/2 feet long. Four rods make a chain. These units of measure are used in surveying today in the US (come on, metric!).
Though using perches for distance, medieval people did not, however, use square-perches as a unit of surface area. Agricultural land would be designated in jornales, that is how many days it would take to cultivate it. This of course varied enormously, depending on whether it was flat or hilly, rocky or smooth, and what kind of plow one was using. But in fact it was a very useful kind of measure, because it suggested how valuable it might be for raising food.
As I've discussed previously, medieval architects did not start with some decision like "this church will be three perches wide." Rather, on the site, they would create a unit measure that would then be used for the elements of a church. So suppose you liked a church with its 5-units-high doorway. You would request the architect to build a church just like it, only (for example) make our unit just a little bigger!
© C. Dale Brittain 2019
For more on medieval life and thought, see my ebook, Positively Medieval: Life and Society in the Middle Ages.
This is very helpful.
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