Did you know that three independently controlled keys are required to open the vault where the official references of the kilogram are stored. The kilogram is an agreement. The reference of the kilogram is the International prototype kilogram or IPK. There are six sister copies stored at the International Bureau of Weights and Measures (known by its French-language initials BIPM) in an environmentally monitored safe in the lower vault located in the basement of the BIPM's Pavillon de Breteuil in Sèvres on the outskirts of Paris. Official copies of the IPK were made available to other nations to serve as their national standards. These are compared to the IPK roughly every 40 years, thereby providing traceability of local measurements back to the IPK.