The gram (alternative British English spelling: gramme; SI unit symbol: g) (Greek/Latin root grámma) is a metric system unit of mass. Gram can be abbreviated as gm or g. Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" (later 4 °C, the so called triple-point of water where ice, liquid, and vapor can undergo phase transformation from any one to any other phase—the reason it was attractive as a standard). However, in a reversal of reference and defined units, a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or 1×10−3 kg, which itself is now defined, not in terms of grams, but as being equal to the mass of a physical prototype of a specific alloy kept locked up and preserved by the International Bureau of Weights and Measures. This is in the tradition by which many customary local reference standard 'stones', lengths (objects) and weights were required to periodically undergo comparison with the official nations standard referents, usually with a particular periodicity defined by the countries statuate laws.