Product Code Database
A unit prefix is a specifier or mnemonic that is added to the beginning of a unit of measurement to indicate multiples or fractions of the units. Units of various sizes are commonly formed by the use of such . The Metric prefix, such as kilo- and milli-, represent multiplication by positive or negative exponentiation 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.
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, centimetre, and centilitre, are widely used; outside the SI, the units hectare, decibel are also common. The unit prefixes are always considered to be part of the unit, so that, e.g., in exponentiation, 1 km2 means one square kilometre, not one thousand square metres, 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-, (from the Greek myriad, mýrioi), double- and demi-, denoting factors of , 2 and respectively, 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.
Units of information are not covered in the International System of Units. Computer professionals have historically used the same spelling, pronunciation and symbols for the binary series in the description of computer memory, although the symbol for kilo is often capitalised. For example, in citations of main memory or RAM capacity, kilobyte, megabyte and gigabyte customarily mean (210), (220) and (230) bytes respectively.
In the specifications of hard disk drive capacities and network transmission , decimal prefixes are used. For example, a 500-gigabyte hard drive holds 500 billion bytes, and a 100-megabit-per-second Ethernet connection transfers data at 100 million bits per second. The ambiguity has led to some confusion and even lawsuits from purchasers who were expecting 220 or 230 and considered themselves shortchanged by the seller. (see Orin Safier v. Western Digital Corporation and Cho v. Seagate Technology (US) Holdings, Inc.). Seagate lawsuit concludes, settlement announced, bit-tech.net To protect themselves, some sellers write out the full term as "".
With the aim of avoiding ambiguity the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998 ( formerly subclauses 3.8 and 3.9 of ). Each binary prefix is formed from the first syllable of the decimal prefix with the similar value, and the syllable "bi". The symbols are the decimal symbol, always capitalised, followed by the letter "i". According to these standards, kilo, mega, giga, et seq. should only be used in the decimal sense, even when referring to data storage capacities: kilobyte and megabyte denote one thousand and one million bytes respectively (consistent with the metric system), while terms such as kibibyte, mebibyte and gibibyte, with symbols KiB, MiB and GiB, denote 210, 220 and 230 bytes respectively.
The ascending prefixes peta (5) and exa (6) are based on the Greek-derived "penta" (5) and "hexa" (6). The largest prefixes zetta (7), and yotta (8) and, similarly, the descending prefixes zepto (−7) and yocto (−8) are derived from Latin Proceedings of the 19th CGPM (1991), page 80. "septem" (7) and " octo" (8) plus the initial letters "z" and "y". The initial letters "z" and "y" appear in the largest SI prefixes. They were changed because of previously proposed ascending hepto (Greek "hepta" (7)) was already in use as a numerical prefix (implying seven) and the letter "h" as both SI-accepted non-SI unit (hour) and prefix ( hecto 102), the same applied to "s" from previously proposed descending septo (i.e. SI unit "s", seconds), while "o" for octo was problematic since a symbol "o" could be confused with zero."The names zepto and zetta suggest the digit seven sept (seventh power of 103) and the letter 'z' replaces the letter 's' to avoid the duplicate use of the letter 's' as a symbol. The names yocto and yotta are derived from octo, which suggests the number eight (eighth power of 103); the letter 'y' is added to avoid the use of the letter 'o' as a symbol because of the possible confusion with the digit zero." Resolution 4 of the 19th CGPM (1991)
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