Explanation

Since 1879, the kilogram has been defined as the weight of “Le Grand K”, a cylinder of platinum and iridium weighing a little over 2 pounds and kept in a locked vault in Paris. For more than a century, the kilogram (kg) — the fundamental unit of mass in the International System of Units (SI) — was defined as exactly equal to the mass of a small polished cylinder, cast in 1879 of platinum and iridium.