The Metric System
Before the French Revolution, it was common for units of weight and measurement to vary not only between countries, but between different regions, or even towns, within the same country. In line with the Enlightenment and the Age of Reason, the Revolution sought to bring order to that confusing and conflicting systems of weights and measurements.
The need for a uniform system was obvious to educated people, scientists, and merchants throughout Europe. However, the power of inertia was strong: most people had grown up with and were used to their own traditional systems of weight and measurement, irrational as they might have been. Such systems might have been confusing to outsiders, and might have required conversions that added unnecessary complexity to trade between towns, regions, and countries. However, they were not confusing to most people who used them in their day to day lives, and who were thus loathe to replace them with a new system that they would have to learn from scratch.
Because of that inertia and reluctance to change, it was only within the context of a major revolutionary upheaval that upended and swept all before it that such a major change could be considered. Luckily, the French Revolution was just such a major upheaval. The Metric System was first proposed in the National Assembly in 1791. Legislation was passed in 1795, and in 1799, the first standard meter bar and kilogram bars were adopted.
Unlike the panoply of traditional units based on the human body, such as feet or thumbs that varied from person to person, the Metric System used the Earth itself as the measuring stick. A meter was defined as one ten-millionth of the distance between the North Pole and the Equator. A liter was the volume of one cubic decimeter. A kilogram was the weight of a liter of pure water. It was simple, elegant, and based on the decimal system, lending itself to easy calculations.
It took a while for the new system to take hold, and it was not made compulsory in France until 1837. Indeed, while the Metric System was French in origin, France was not the first country to require its use: the Netherlands, Belgium, and Luxembourg made the system compulsory in 1820. However, it was such a rational and simple system, superior to all others, that its use could not help but spread.
By 1850, a global economy was developing, the need for global uniformity in weights and measurements became obvious, and so a movement began in favor of an international system. The Metric System was the only viable option: its only competitor was the British Imperial System, but in addition to being confusing, it was too closely associated with the British Empire for those outside it. In 1875, all industrialized countries, with the exception of Britain, signed a treaty that established the International Bureau of Weights of Measures. It presides to this day over the International System of Units, which governs global measurements and weights.