A kilogram, often denoted by kg, is one thousand grams, and is one of seven SI base units. It is a unit of mass. The SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium and kept at the Bureau International des Poids et Mesures (International bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes. The international protoype kilogram was made in the 1880s.