A tonne (t) or metric ton, also referre to as a metric tonne or tonne métrique, is a measurement of mass equal to 1,000 kilograms. It is not an SI unit but is accepted for use with the SI. The proper SI unit for a tonne would be a "megagram" (Mg, see SI prefix), but this term is rarely used in practice. Though the spelling tonne predates the introduction of the SI system in 1960 (it has been used in France for centuries, where it comes from), it is now used as the standard spelling for the metric mass measurement in some English-speaking countries. In the United States the correct term is metric ton. The comparable imperial and US customary units are spelled ton in English.