A pound of sugar. A yard of silk. A tael of silver. A liter of water.
We take our units of measurement for granted. They’re pervasive in our lives, infiltrating everything we do; you can’t bake a cake, build a house, or drive a car go without someone, somewhere, having done the work of weighing and measuring, making certain that everything will fit together and produce the right results when combined.
But where do these measurements come from? And how do we ensure they remain consistent?
All units are, at their heart, arbitrary. Sure, one unit may be based on another, but all of them ultimately trace back to somebody making the decision that this was the weight, distance, volume, etc. that other things would henceforth conform to. Since lots of people made those decisions, and they didn’t consult with each other, what resulted was a melanges of units — some of them going under the same name, but of incompatible sizes. An ell used to be defined as the distance from the elbow to the tips of the fingers; whose arm are we using for this? In London it was the king’s arm . . . but some kings are going to have longer arms than others, meaning that the length of an ell fluctuates over time. And people in other parts of England had to use other arms.
Which, it turns out, is really bad for commerce. Your mistress tells you to go out and buy ten ells of wool; you buy that much, but when you deliver it to the lady of the house, she complains that you overspent and brought her eleven ells. You purchase three what you think is three pounds of flour, but it winds up being not enough for your journey, because by your reckoning you received only two and a half pounds. Sometimes that’s because the miller deliberately shorted you — people cheating on their weights and measures used to be a constant problem — but sometimes it’s simply because none of you agree on your units.
Because of this, it isn’t surprising that governments have been getting into the business of regulating weights and measures for a long, long time. Going back for millennia, we find evidence of kings taking an interest in this matter (and punishing people who deliberately cheated). More is at stake than just the ability of common people to sew dresses or bake bread: if you’re outfitting an army, you need to be able to trust that you’ll get the quantities you need in the proportions you require. Otherwise you might wind up with arrows too short for effective shooting, or boots too small for your soldiers’ feet. Military demand is why the thirty-fifth clause of the Magna Carta decreed that there would be one standard apiece for weights, volumes, and cloth widths in England, rather than the mess of local standards that prevailed before.
But how do you ensure those standards remain accurate? We all know that copying introduces errors; if somebody uses my yardstick to cut their own, and then someone else uses that second one as a model, rinse and repeat, eventually you’ll wind up with yardsticks that differ in length by whole inches. Especially if, along the way, there’s an unscrupulous cloth merchant who wants to make more money off a bolt of cotton . . .
Until quite recently, the only way to guarantee actual standardization was to have a single, authoritative model against which all others could be compared. In the royal treasury, alongside the gold and jewels — and maybe decorated with gold and jewels itself — you would have the One True Yardstick, the One True Gallon Jug, and so forth. A well-organized state might periodically check a set of certified measuring devices against those definitive models, then send out officials with the certified copies to vet the ones being used across the realm. You can’t check everybody’s yardstick, of course . . . but you can do surprise inspections, or particularly scrutinize someone accused of false dealing. Error still creeps in, but it gets corrected.
I say this was the case until quite recently because the scientific community has been hard at work on fixing this problem. One by one, they’ve found ways to describe the basic units of everything from mass to luminous intensity in terms of scientific constants. As arcane as this might seem, it’s quite necessary: when the International Prototype of the Kilogram was checked against various national prototypes in 1948 and 1989 (it was first made in 1889), researchers found that it had somehow become lightweight compared to its copies. Not by a lot — fifty micrograms or so — but when science depends on a high degree of precision, that much fluctuation is alarming. As of 2019, though, all the basic units can be defined according to universal constants, rather than the low-tech method of keeping a hunk of metal in a vault.
When units of measurement play a role in fiction, it’s usually in the form of a fantasy author dropping in an archaic unit, like having a character walk three leagues (roughly defined as how far a person can walk in an hour, or somewhat more precisely as three miles — but remember that when they’re at sea, a league is three nautical miles, which are longer!). Or if the setting is meant to evoke a particular culture, the text might use terms specific to that culture, like the Chinese li (officially standardized now to half a kilometer, but fluctuating considerably throughout history). Only rarely do I see a story go the route of the online game Final Fantasy XIV, where distances are described in ilms, yalms, and malms, and the audience has to rely on context to guess that these are roughly equivalent to inches, yards, and miles. Amusingly, though, the encyclopedia for the game apparently mentions that an ilm was based on the thumb of a Hyur, and when that thumb was severed in an argument, replicas were cast and distributed to help standardize measurements throughout the realm — so someone at Square Enix was thinking it through!
But this isn’t just about what words you drop into the text. There’s cultural weight to it, too — witness the fact that the United States still refuses to adopt the metric system used by every other country in the world except Burma and Liberia. As an empire’s territory expands, it will usually impose its standards on the peoples they conquer or assimilate; neighboring peoples may voluntarily adopt those standards, because agreeing on terms facilitates trade. Even after the empire recedes, its units often remain . . . but without oversight, they start to diverge, creating disputes or subtleties of negotiation around whose dirham standard will be used to weigh the sale. And even when a new system comes in, old units can persist in specific contexts. Britain may be metric, but Brits still buy a pint of beer at the pub instead of a half-liter, and many of them still describe their weight in terms of stones (a stone equaling fourteen pounds).
I’ve said before that culture is often a palimpsest. This is one of the countless places an author can show the lingering influence of the past, or the layering effects of cross-cultural contact, adding depth to a setting simply through their choice of words. Or you run with an idea like my sister’s and write a heist story based around sneaking in to shave a bit off the International Prototype of the Kilogram . . .