The Metric System — A History of Measurement

Simon Stevin published De Thiende ("The Tenth"), the pamphlet that established modern decimal-fraction notation in Europe and argued that decimal coinage, weights, and measures were inevitable. Its arithmetic became the mathematical precondition for a coherent decimal measurement system two centuries later.

Location: Bruges, Flanders

The abbot Gabriel Mouton proposed a decimal unit of length defined as a fraction of the Earth's circumference (the milliare), with decimal multiples and submultiples — concepts eventually used in the metre's definition. Around the same period, proposals by Christopher Wren and Christiaan Huygens tied units to pendulum periods.

Location: Lyon, France

Following a proposal presented by Talleyrand, the French National Assembly, on the king's instruction, charged the Academy of Sciences — a commission including Lagrange, Laplace, Lavoisier, and Monge — with defining a standard of weights and measures "for all people, for all time," derived from nature rather than royal authority or local custom.

Location: Paris, France

Astronomers Pierre Méchain and Jean-Baptiste Delambre began a seven-year geodetic survey of the Dunkirk-to-Barcelona meridian arc, measuring through the Revolution's wars to determine the Earth's quadrant. On March 19, 1791 the Academy had defined the metre as one ten-millionth of that quadrant — a standard intentionally independent of any artifact or monarchy.

Location: France and Spain

The law of 18 Germinal, Year III, formally established the metric system in France, defining the metre, the gram (mass of a cubic centimetre of water), and decimal multiples with Greek and Latin prefixes. Popular resistance remained strong, and Napoleon's 1812 mesures usuelles partially reverted to customary units until the July Monarchy restored metric exclusively from 1840.

Location: Paris, France

Seventeen nations, including the United States, signed the Metre Convention (Treaty of the Metre), establishing the International Bureau of Weights and Measures (BIPM) and the General Conference on Weights and Measures (CGPM) to maintain common platinum-iridium standards. The treaty converted a French national reform into the basis of world measurement; it is commemorated each year as World Metrology Day.

Location: Paris, France

Electrical engineer Giovanni Giorgi demonstrated that a coherent system of units incorporating electricity required a fourth base unit alongside length, mass, and time. His MKS-based proposal became the structural foundation on which the modern SI was later built.

Location: Rome, Italy

The 11th CGPM launched the Système international d'unités (SI) with six base units — metre, kilogram, second, ampere, kelvin, and candela — unifying physical measurement under one coherent framework; the mole joined as the seventh base unit in 1971. The same conference redefined the metre in terms of the wavelength of krypton-86 radiation, beginning the shift from artifacts to physical constants.

Location: Paris, France

The 17th CGPM redefined the metre as the distance light travels in a vacuum in 1/299,792,458 of a second, fixing the speed of light as an exact constant. With the second already defined by caesium atomic clocks, the last human-made length standard was retired; the United States, Liberia, and Myanmar remained the only countries not formally adopting the metric system as their primary system, though US science, medicine, and industry use it extensively.

Location: Paris, France

The 26th CGPM adopted a revision defining all seven SI base units through fixed physical constants — including the Planck constant for the kilogram — eliminating dependence on physical artifacts entirely. The International Prototype Kilogram, a platinum cylinder kept under vacuum at the BIPM since 1889, was retired, completing the system's two-century migration from the Earth's meridian to the constants of the universe.

Location: Versailles, France