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.

Read: Why doesn’t the United States use the metric system?

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.