The modern kilogram has its origins in the pre-French Revolution days of France. Louis XVI created a Consultative Commission for Units to devise a new decimal-based system of measurement. This royal commission, which included such aristocrats as Antoine Lavoisier, founded the very beginnings of the “metric system”, which later evolved into the contemporary International System of Units (SI).
On 7 April 1795, the “gramme”, upon which the kilogram is based, was decreed to be equal to “the absolute weight of a volume of pure water equal to a cube of one hundredth of a metre, and at the temperature of the melting ice”. Although this was the definition of the gram, the regulation of trade and commerce required a “practical realisation”: a single-piece, metallic reference standard that was one thousand times more massive that would be known as “grave” (symbol G). This mass unit, whose name is derived from the word “gravity”, was used since 1793. Notwithstanding that the definition of the base unit of mass was the gramme (alternatively “gravet”), this new, practical realisation would ultimately become the base unit of mass. A provisional kilogram standard was made and work was commissioned to determine precisely how massive a cubic decimetre (later to be defined as equal to one litre) of water was.
Although the decreed definition of the kilogram specified water at 0 °C — a highly stable temperature point — the scientists tasked with producing the new practical realisation chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C. They concluded that one cubic decimetre of water at its maximum density was equal to 99.92072% of the mass of the provisional kilogram made earlier that year. Four years later in 1799, an all-platinum standard, the “Kilogramme des Archives”, was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, to the mass of cubic decimetre of water at 4 °C. The kilogram was defined to be equal to the mass of the Kilogramme des Archives and this standard stood for the next ninety years.
Note that the new metric system did not come into effect until after the French Revolution, when the new revolutionary government captured the idea of the metric system. The decision of the Republican government to name this new unit the “kilogramme” had been mainly politically motivated, because the name “grave” was at that time considered politically incorrect as it resembled the aristocratic German title of Graf, an alternative name for the title of Count that, like other nobility titles, was inconsistent with the new French Republic notion of equality (égalité). Accordingly, the name of the original, defined unit of mass, “gramme”, which was too small to serve as a practical realisation, was adopted and the new prefix “kilo” was prefixed to it to form the name “kilogramme”. Consequently, the kilogram is the only SI base unit that has an SI prefix as part of its unit name.