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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. The prefixes of the metric system, such as ''kilo'' and ''milli'', represent multiplication by powers of ten. In information technology it is common to use binary prefixes, which are based on powers of two. Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognised by standards organisations. ==Metric prefixes== (詳細は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. 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. In mathematical contexts, the unit prefixes are always considered part of the unit, so that, e.g., in exponentiation, 1 km2 means one square kilometre not one thousand square metre and 1 cm3 means one cubic centimetre not one hundredth of a cubic metre. In general, prefixes are used with any metric unit, but may also be used with non-metric units. Some combinations, however, are more common than others. The choice of prefixes for a given unit has often arisen by convenience of use and historical developments. Unit prefixes that are much larger or smaller than encountered in practice are seldom used, albeit valid combinations. 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-'',〔(【引用サイトリンク】title=H.R. 596, An Act to authorise the use of the metric system of weights and measures )〕 (from the Greek μύριοι, ''mýrioi''), ''double-'' and ''demi-'', denoting a factors of , 2 and respectively,〔(histoire.du.metre.free.fr )〕 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 prefix "''myrio-''" was an alternative spelling variant for "''myria-''", as proposed by Thomas Young. 〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Unit prefix」の詳細全文を読む スポンサード リンク
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