At press time, only three of the world's countries don't use the metric system: the United States, Myanmar and Liberia. But it didn't have to be this way.

On this day in 1866, the Metric Act was passed by the Senate. The law, which was intended "to authorize the use of the metric system of weights and measures," was signed by then-President Andrew Johnson the next day. It provided a table of standardized measurements for converting between metric and the commonly used American system that could be used for trade.

The Metric Act doesn't require Americans to use the metric system, but it did legally recognize the then-relatively-new system. It remains law–although it has been substantially amended over time–to this day, writes the US Metric Association. It was just the first in a number of measures leading to the United States’ current system, where metric is used for some things, like soda, drugs and even for military use, but not for other things. "Americans' body-weight scales, recipes and road signs," among other examples of everyday use, haven't converted, writes Victoria Clayton for The Atlantic. "And neither has the country's educational system," she writes. This split system exists for reasons, but arguments about how to create a good national standard of measurement go all the way back to 1790.

The USMA is one of a number of voices advocating for America’s full “metrification.” It argues that converting to the International System of Units (the modern form of the metric system, abbreviated as SI) would make international trade simpler. (Technically, the American system known as Imperial is called United States customary units or USCS.) It also argues that the decimalized metric system is simpler to work with.

SI units influence the size of packages (such as 750 ml bottles of wine ) as well as how the package must be labelled. Since 1994, both metric and USCS have been required on commerical packaging under the Fair Packaging and Labeling Act.

“The United States is metric, or at least more metric than most of us realize,” writes John Bemelmans Marciano for Time:

American manufacturers have put out all-metric cars, and the wine and spirits industry abandoned fifths for 75-milliliter bottles. The metric system is, quietly and behind the scenes, now the standard in most industries, with a few notable exceptions like construction. Its use in public life is also on the uptick, as anyone who has run a “5K” can tell you.

America has been creeping towards metrification almost since the country was founded.

“In 1790, the United States was ripe for conversion,” writes David Owen for The New Yorker. At the time, the metric system was a new French invention (SI stands for Systeme Internationale), and adopting a system that departed from the Old World conventions and was based on modern decimalized units seemed like a good fit for the United States.

The French and Americans had supported and conflicted with one another over their revolutions in statehood, Owen writes, and there was some expectation on the part of the French that the country would join them in the measurement revolution as well.

But even though “the government was shopping for a uniform system of weights and measures,” Owen writes, the meter was too new, and too French. Then-Secretary of State Thomas Jefferson originally advocated for the meter, but then discarded the idea. “His beef was that the meter was conceived as a portion of a survey of France, which could only be measured in French territory,” writes Marciano.

In the course of the nineteenth century, though, the meter gained traction again and other countries started to pick up on it. However, by this point in time, American industrialists already ran all of their equipment based on inch units. “Retooling, they argued, was prohibitively expensive,” historian Stephen Mihm told The Atlantic. “They successfully blocked the adoption of the metric system in Congress on a number of occasions in the late 19th and 20th century.”

Add to these arguments America’s nationalist pride and traditional resistance to outside influences, and you have an argument for maintaining the status quo–metric, with a quarter-inch veneer of Imperial.