English: A national prototype kilogram standard mass, number K4, kept by the National Institute of Standards and Technology, USA, one of two which serve as the official standard for defining all units of weight and mass in the United States. It is a polished cylinder made of 90% platinum - 10% iridium alloy, 39 mm (1.5 inches) in diameter and 39 mm high. It is an exact copy of the international prototype kilogram kept at the International Bureau of Weights and Measures (Bureau International des Poids et Measures) in Sevres, France It was one of 40 copies which were presented by France in 1884 to different nations as national standards. This copy, K4, serves as a secondary comparison standard, while its twin, K20, serves as the primary standard. In 1884, it differed in mass from the prototype kilogram by about 75 micrograms. Alterations to image: Removed aliasing artifacts (stripes) due to scanning of halftone photo using GIMP FFT filter.