The kilogram is the last remaining base unit of the SI that is still defined by a material artefact.

The international prototype is kept with its six official copies in a vault at the BIPM.

The international prototype was manufactured in the 1880s of an alloy of 90 % platinum-10 % iridium. Four of the six official copies date from the same period.

In addition, copies of the international prototype have been manufactured by the BIPM for use as 1 kg national prototypes. The first of these were distributed in 1889. Since the 1880s the BIPM has produced more than eighty 1 kg prototypes in Pt/Ir.

The BIPM also maintains a number of 1 kg copies for current use.