A unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units.Units of various sizes are commonly formed by the use of such prefixes.
Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognised by standards organisations.
The prefixes of the metric system precede a basic unit of measure to indicate a decadic multiple and fraction of a unit.
Each prefix has a unique symbol that is prepended to the unit symbol.
Some of the prefixes date back to the introduction of the metric system in the 1790s, but new prefixes have been added, and some have been revised.
In most contexts only a few, the most common, combinations are established.
For example, prefixes for multiples greater than one thousand are rarely applied to the gram or metre.
Some prefixes used in older versions of the metric system are no longer used.
The prefixes myria-, were parts of the original metric system adopted in France in 1795, but they were not retained when the SI prefixes were agreed internationally by the 11th CGPM conference in 1960.
The International Bureau of Weights and Measures has standardised twenty metric prefixes in resolutions dating from 1960 to 1991 for use with the International System of Units (SI).
Although formerly in use, the SI disallows combining prefixes; the microkilogram or centimillimetre, for example, are not permitted.
Prefixes corresponding to powers of one thousand are usually preferred, however, units such as the hectopascal, hectare, decibel, centimetre, and centilitre, are commonly used.