The Systeme International d’Unites (SI) was adopted in 1960 after an international convention of 50 member states including all the major industrialized nations. The SI system defines seven fundamental units that are dimensionally independent. They are the second (s), metre (m), kilogram (kg), ampere (A), kelvin (K), mol (Mol) and candela (Cd). All can be determined by suitably equipped laboratories except the kilogram, which requires reference to a standard platinum/iridium ingot. The fundamental units can be combined algebraically to create derived units. Derived units are expressed as multiplications or divisions of fundamental units and may be named. Examples are the newton for force and the Hertz for frequency. These may be combined to derive further units. The SI system also derives prefixes for multiple and submultiples of SI units to avoid very large or small numbers; and defines the correct usage of SI notation. In the UK, the National Physical Laboratory (NPL) is responsible for national standards in measurement and is a member of the conference maintaining the SI units.