This platinum-iridium cylinder at the National Bureau of Standards is the primary standard kilogram for all metric measurements of mass in this country. It was furnished to the US by the International Bureau of Weights and Measures in accordance with the metric treaty of 1875. Recomparison with the international standard in 1937 showed that the US standard had changed by only one part in 50 million during approximately 50 years. Known as Kilogram No. 20, it is 39mm in diameter and 39mm high, and is made of an alloy containing 90% platinum and 10% iridium.