Diplomats from seventeen countries signed the Convention du Mètre in Paris, creating the International Bureau of Weights and Measures and committing to a unified metric standard for global trade and science. The treaty established physical prototype standards for the meter and kilogram, housed near Paris, and created the governance structure that still oversees the International System of Units (SI) today. World Metrology Day is celebrated annually on this date in commemoration.
On May 20, 1875, representatives of seventeen nations gathered in Paris to sign the Convention du Mètre, a treaty born from the chaos of incompatible measurement systems that had long hampered international trade, science, and diplomacy. Before this agreement, a 'pound' or a 'foot' might mean something entirely different depending on which country—or even which city—you stood in, creating friction for merchants, engineers, and scientists alike.
The treaty created the Bureau International des Poids et Mesures (BIPM), headquartered in Sèvres outside Paris, tasked with maintaining physical prototype standards: a platinum-iridium bar defining the meter and a cylinder defining the kilogram, both kept under controlled conditions and periodically compared against national copies distributed to signatory countries. This system, remarkably, remained the backbone of global measurement for over a century.
The convention's legacy is almost invisible precisely because it succeeded so thoroughly: nearly every scientific measurement made today, and most of the world's commerce, relies on the International System of Units that grew from this 1875 agreement. In recognition, the world now marks May 20 as World Metrology Day, and the last vestiges of the original physical prototypes were finally retired in 2019 when the kilogram was redefined using fundamental physical constants rather than a metal cylinder in a French vault.