to indicate multiples of a second greater than 1; the non-SI units of minute , hour and day are used instead. On the other hand, prefixes are used for multiples of the non-SI unit of volume, the litre (l, L) such as millilitres (ml). [ 4 ] The base units used in the metric system must be realisable , ideally with reference to natural phenomena rather than unique artefacts . Each of the base units in SI is accompanied by a mise en pratique [practical realisation] published by the BIPM that describes in detail at least one way in which the base unit can be measured. [ 11 ] Where possible, definitions of the base units were developed so that any laboratory equipped with proper instruments would be able to realise a standard without reliance on an artefact held by another country. In practice, such realisation is done under the auspices of a mutual acceptance arrangement (MAA). [ 12 ] In the original version of the metric system the base units could be derived from a specified length (the metre) and the weight [mass] of a specified volume ( 1 ⁄ 1000 of a cubic metre) of pure water. Initially the de facto French Government of the day, the Assemblée nationale constituante , considered defining the metre as the length of a pendulum that has a period of one second at 45°N and an altitude equal to sea level . The altitude and latitude were specified to accommodate variations in