The three fundamental units in physics, which form the basis of the c.g.s. system, are the centimetre, the unit of length; the gram, the unit of mass; and the second, the unit of time. Material standards of length and mass are provided by the International Prototype Metre and Kilogram, preserved at the International Bureau of Weights and Measures at Sevres, near Paris. Any length or mass can be compared either directly or indirectly with sub-standards, which have themselves been compared with the actual prototype standards.