The metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not affect its day-to-day use. Over the last two centuries, different variants have been considered the metric system.

Since the 1960s the International System of Units ("Système International d'Unités" in French, hence "SI") has been the internationally recognised standard metric system. Metric units are widely used around the world for personal, commercial and scientific purposes. A standard set of prefixes in powers of ten may be used to derive larger and smaller units from the base units.

According to the US CIA World Factbook in 2006, the International System of Units is the official system of measurement for all nations except for Myanmar, Liberia, and the United States. However, a number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom — where the Traffic Sign Regulations only allow distance signs displaying imperial units (miles or yards) — or Hong Kong. Also, other standardised measuring systems other than metric are still in universal international use, such as nautical miles and knots in international aviation.

One goal of the metric system is to have a single unit for any physical quantity; another important one is not needing conversion factors when making calculations with physical quantities. All lengths and distances, for example, are measured in meters, or thousandths of a meter (millimeters), or thousands of meters (kilometer), and so on. There is no profusion of different units with different conversion factors, such as inches, feet, yards, fathoms, rods, chains, furlongs, miles, nautical miles, leagues, etc.

Multiples and submultiples are related to the fundamental unit by factors of powers of ten, so that one can convert by simply moving the decimal place: 1.234 metres is 1234 millimeters, 0.001234 kilometres, etc. The use of fractions, such as 2⁄7 of a metre, is not prohibited, but uncommon, as it is generally not necessary.

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