which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten, so for example where the unit of length is the metre; a distance of 1 m is 1,000 millimetres, or 0.001 kilometres. Current practice Metrication is complete or nearly complete in almost all countries. US customary units are heavily used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma. U.S. units are used in limited contexts in Canada due to the large volume of trade; there is also considerable use of imperial weights and measures, despite de jure Canadian conversion to metric. A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom – whose road signage legislation, for instance, only allows distance signs displaying imperial units (miles or yards) – or Hong Kong. Metric units are virtually always used in science, frequently in the military, and partially in industry in the United States, but customary units are primarily used in households. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than grains, are used for medications. Some other non-SI units are still in international use, such as nautical miles and knots in aviation and shipping. Metric system Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries. Multiples and submultiples of metric units are related by powers of ten and their names are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units. In the early metric system there were two base units, the metre for length and the gram for mass. The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units. Mesures usuelles (French for customary measurements) were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was