Watercolour pencil on Arches watercolour paper, 76 x 56 cm

The kilogram was the last of the SI units defined by a physical object. The International Prototype of the Kilogram (IPK), also known as le grand K is this artefact. It is a cylinder of metal, a platinum-iridium alloy, housed in the International Bureau of Weights and Measures (BIPM) outside Paris.

Le grand K was the defining mass by which all kilograms were measured between 1889 and May 2019. The kilogram is now defined in terms of the second and the metre and is based on fixed fundamental constants in nature.