A copy of Le Grand K, the cylinder that currently defines the base unit for metric massJacques Brinon / AP

Meters, kilograms, degrees Celsius. To most Americans, these units of measurement are little more than funny inconveniences on trips abroad. To scientists, they’re the very standards that allow for meaningful comparisons of experiments. But to historians of metrology—the study of measurement—those innocuous-looking units are something else entirely: the culmination of a long, fraught battle against tyranny.

In centuries past, lords and ministers in Europe and beyond often manipulated units to steal land, fix commodities markets, cheat peasants out of goods, and wring extra labor and taxes out of them. Fluctuating units also helped concentrate power in the hands of despots. Using a ruler or scale might not seem like a political act, but according to the late Polish historian Witold Kula, in his book Measures and Men, units of length, weight, and volume in the past were both “instrument[s] of asserting class privilege” and “the center of a bitter class struggle” dating back several millennia.

Next month, the 26th meeting of the General Conference on Weights and Measures will likely adopt new methods of defining four basic international units, including the kilogram. The kilogram is the very last international unit based on a human-made object: a platinum-iridium cylinder in an underground, high-security vault in Paris. This cylinder has, by definition, a mass of 1.0000 … kilogram, to as many decimal places as you like. But it also has a problem: On an infinitesimal level, its mass has changed over the years, causing all sorts of complications.

The new definition will be based on universal constants of nature—quantities that don’t change, like the charge of an electron—which will allow any lab around the world to reproduce a one-kilogram mass on its own by rigging up the right equipment. (You can read about the new definitions here and here.) But far from simply tidying up a few loose metrological ends, these new definitions will also close a chapter on our past—an era when units of measure weren’t just humdrum tools, but a real frontier in the struggle for equality.

Stories about this new, universal kilogram usually portray it as a triumph of progress. Look how far we’ve come! And it’s true that the kilogram and other components of the metric system (the official standard everywhere in the world save Burma, Liberia, and the United States) do seem eminently rational compared to the metrological chaos of yesteryear. Way back when, the same units often differed significantly from village to village. A “bushel” in one town wasn’t the same as a “bushel” in another. In medieval Geneva, a “pound” could be 15, 16, or 18 ounces, depending on the goods being sold. Cloth wholesalers might use one length, cloth retailers another, and fishermen measured the width of their nets using one unit and the breadth using another.

Units were often divvied up in maddening ways as well. The metric system is based on units of 10: There are 10 millimeters in a centimeter, 10 centimeters in a decimeter, 10 decimeters in a full meter, and so on. This makes bouncing up and down the scale easy. In contrast, consider this 18th-century French system, as described in an old questionnaire that Kula quotes in his book: “To measure dry goods we use the resal and its divisions. The resal consists of eight units called bichot, each of these dividing into six pots. The pot divides into two pintes, the pinte into two chopmes, the chopine into two setters, and the setter into three verres.” As Kula writes, they eventually ended up with one-1,152nd of the resal.

The old units did have a few advantages. Yes, 1,152 is an ugly number. But unlike 10, you can divide it into thirds, quarters, sixths, eighths, or twelfths without decimals or fractions, which made commerce easier for commoners. Variable units also gave the economic system needed flexibility. In centuries past, commodity prices were often fixed and immutable: You always paid 25 shillings or whatever for a bushel of grain, and “that [price] may not be altered by man other than sinfully,” as Kula puts it. So the only way to respond to market conditions was by varying the size of the bushel. Prices could remain fixed, in other words, as long as quantity fluctuated.

Still, we shouldn’t romanticize ye olde units, because that flexibility also enabled chicanery and abuse. Peasants often paid tributes to feudal lords in the form of goods—X baskets of grain a year, or Y yards of cloth. Those numbers were fixed by custom, and lords couldn’t raise them without incurring hellfire and damnation. So they cheated their vassals in other ways. A lord might loan you five regular baskets of grain to get through a drought, but demand repayment in larger ones. Greedy landlords could goose an extra 25 percent from their subjects this way. (This wasn’t just a European problem. Chinese landlords pulled the same shenanigans, and in Burma, the peasants under one egregious landowner nicknamed the basket he used for receiving tributes “the cart-breaker.”)

Another shady practice involved land measurement. European lords often measured fields with a unit of distance called an ell (short for an elbow or forearm, the upper-body equivalent of feet). Serfs might have to plow or weed a certain number of square ells to fulfill their obligations. But in places where that number was fixed by custom, lords could always employ an extra-long ell to expand the area and extract more work.

Even the humble alehouse couldn’t escape cheating. After a hard week’s labor, drinkers wanted to make sure they were getting a full volume of booze. So people insisted that tavern keepers compare their serving cups to a community standard. Some towns even required barkeeps to prepare wooden labels attesting to a “just measure” of spirits. These labels were then attached to every cup with strings, which dangled down whenever people raised their wrists to quaff. And woe betide anyone who violated this custom: Many Polish churches had murals depicting sinners suffering in hell, and one prominent victim was often an innkeeper who shorted his patrons on vodka.

Overall, as the anthropologist James C. Scott remarks in Seeing Like a State: How Certain Schemes to Improve the Human Condition Have Failed, “every act of measurement was an act marked by the play of power relations”—usually in favor of the nobles. As a result, most peasants grew to hate fluctuating units and wanted to establish standards: fixed volumes, fixed lengths, and fixed weights.

Oddly enough, peasants found an ally in absolute monarchs, who also opposed fluctuating units. They did so partly in support of their subjects. Monarchs sat above lords in the political hierarchy, and playing the people against the lords was smart politics. But even more than that, as Scott emphasizes, metrological chaos made kingdoms a nightmare to administer. Without consistent units, a king’s ministers had a devil of a time determining how much grain or cloth or iron a region produced. This in turn made levying taxes almost impossible. Even drawing up conversion charts to compare one region’s units with another’s was a Herculean task—and often futile, given how frequently units shifted.

Beyond being a bureaucratic mess, haphazard units led to social strife. Kings would inadvertently charge some places more in taxes than they could afford, while letting other places off easy—a sure recipe for resentment and rebellion. Even worse, poor knowledge of the kingdom’s food supply could exacerbate famines and droughts, threatening state security. And unable to levy fair taxes on goods, monarchs often relied on other sources of revenue—like selling offices and titles—that bred corruption.

For these reasons, rulers throughout history—Charlemagne, Alexander the Great, Louis XIV, Moses—all tried, in vain, to impose standard units across their kingdom. (In the Old Testament, God tells Moses, “You shall do no unrighteousness in judgment regarding measures in length, weight, or quantity. You shall have honest balances, honest weights … ” The Koran condemns such cheating, too.) Only in the late 1700s did a few places, especially France, show any success. The reason is complex, but a few factors stand out. First, the power of feudal-style lords—who considered it a sovereign right to change units at their whim—began to wane. Second, an increasingly powerful merchant class began to push for standard measures to grease long-distance trade. Third, natural philosophers—the precursors of modern scientists—developed the metric system during the Enlightenment and promoted it as a rational alternative to metrological chaos.

Far from being the driving force behind standard units, scientists were probably the weakest leg, often dismissed as starry-eyed utopians. Only the upheavals of the French Revolution—and the subsequent rise of Napoleon, who was strong enough to impose the metric system on his empire—gave the idea of standard units real traction and allowed the practice to spread, however fitfully, across Europe. As Kula remarks, “The meter followed the flag.”

(Indeed, the impotence of natural philosophers is most clear in the reforms they didn’t achieve. Beyond weights and measures, they also proposed reforming the clock and calendar. Under so-called metric time, days would last 10 “hours,” hours would last 100 “minutes,” and minutes would last 100 “seconds.” The leaders of the French Revolution actually implemented this system in 1793, alongside the meter and kilogram. But most people loathed the new clock, and without the support of commoners, metric time wilted and died.)

So did the poor finally get a fair shake under the new, standardized system of weights and measures? Of course not. The powerful found new ways to cheat the powerless, and in some cases, the metrological revolution weakened the political power of the masses. By helping monarchs administer their kingdom, standard units gave them a way to collect more taxes and thereby bolster their strength. With more accurate and stable land measures, they could also produce better maps of the kingdom. This allowed them to surveil their subjects more effectively and snatch up all unclaimed territories for the crown.

In the short term, then, the natural philosophers’ brilliant new units probably helped concentrate power in the hands of kings and despots. “As in many other walks of life, so in metrology, the state authority of enlightened absolutism had little difficulty in securing the cooperation of men of learning,” Kula writes.

Still, in the long run, we’ve all benefited. Establishing standard units cut down on old forms of abuse, and the efficient administration of kingdoms also boosted public-welfare programs like health and sanitation projects. If nothing else, the metric system is the foundation of modern science and technology, which has lifted innumerable people out of poverty and saved countless lives.

It’s hard to imagine nowadays that mundane units of measure sometimes, as Kula puts it, “horrified and terrified the people.” But like with vaccines and clean drinking water, it’s often the little, taken-for-granted innovations that improve our life the most. So when the new definition of the kilogram is anticipated to officially take effect (on World Metrology Day, May 20), take a moment to reflect on how far we’ve come. Or better yet, raise a glass of wine or beer—knowing full well that you’ll get your “just measure”—and remember all of our poor, put-upon ancestors who weren’t so lucky.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.