Outside number 36 of the Rue de Vaurigard in Paris is one of only two remaining meter standards installed in the late 18th century to get the people of Paris used to the new metric system, introduced in 1795.
The metric system became mandatory in France in 1799, and spread across the world. Until then, people had been using variable units of measure for length, weight and time that made communication difficult. If a tradesman sold you a “foot of fabric”, he basically got to decide how long a foot was to him, or use a local definition which might not correspond to the use of length at his destination market town.
In 1960, the metric system was further formalized as the International System of Units (SI system). That system has a built-in clause that says that they will always use the best available standard to set each unit.
In the early days of the metric system, the definition of a “meter” was one ten-millionth of a quarter of the meridian, measured between a belfry in Dunkirk to the fortress of Montjuïc in Barcelona. That measurement alone was an epic science travel adventure. Quoting Wikipedia:
“The task of surveying the meridian arc fell to Pierre Méchain and Jean-Baptiste Delambre, and took more than six years (1792–98). The technical difficulties were not the only problems the surveyors had to face in the convulsed period of the aftermath of the Revolution: Méchain and Delambre, and later Arago, were imprisoned several times during their surveys, and Méchain died in 1804 of yellow fever, which he contracted while trying to improve his original results in northern Spain.”
Using distances on a (slowly moving) Earth as standards is not very feasible, so in 1889 the reference meter became a default bar of metal, several prototypes of which were distributed to different countries to use to calibrate their local meters. This was replaced in 1960 by a standard derived from the wavelength of the orange-red line of krypton-86. When a unit of length is discovered that is more accurate and stable, that will replace the reference for the meter.
Although some countries still use imperial units of measurement, the SI system is formally used in most countries of the world, with three current exceptions: Myanmar, Liberia, and the United States. In 1975, the USA did sign a “Metric Conversion Act”, acknowledging that the rest of the world seemed to be using the system, and they needed to be able to use it and deal with it appropriately. Meanwhile, Myanmar and LIberia, like the USA, use metrics for international trade purposes, and Myanmar recently announced that it will switch officially to the metric system.
Even countries that do formally use the SI system may still casually use old units, but these are becoming more problematic now that people communicate globally. A pint in the UK is more than in the USA, for example – and not just in cost at the pub! The British Imperial pint is 568 mL, but an American (liquid) pint is only 473 mL.
Glasses of beer usually stay within their country. The real problem is measurements used in recipes. If you find a recipe online and it tells you to add a “cup” of something, how much is that? It could be anything between 200 – 284 mL, depending on where your recipe originated and whether the creator of the recipe used “customary” or “legal” cups. (And why do so many recipes use volume measurements for dry ingredients anyway? Get a scale!)
It’s exactly these sorts of confusions that led the French to introduce a standardized system in the 18th century, and its near-global adaptation and scientific consensus means that protocols for science experiments are more accurate than recipes for cupcakes.