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Historical context of the SI meter kilogram second ampere kelvin mole candela history of SI Unit of mass (kilogram) Abbreviations: CGPM, CIPM, BIPM        At the end of the 18th century, a kilogram was the mass of a cubic decimeter of water. In 1889, the 1st CGPM sanctioned the international prototype of the kilogram, made of platinum-iridium, and declared: This prototype shall henceforth be considered to be the unit of mass. The picture at the right shows the platinum-iridium international prototype, as kept at the International Bureau of Weights and Measures under conditions specified by the 1st CGPM in 1889.        The 3d CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the word "weight," confirmed that: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.   Return to SI Units Background