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An SI prefix is a prefix which can be applied to any unit of the International System of Units (SI) to give subdivisions and multiples of that unit. Many of these prefixes, and the very idea of using prefixes for this purpose, predate the introduction of the SI in 1960, so they are also quite properly used with many non-SI units. As part of the SI system they are officially determined by the Bureau International des Poids et Mesures.
OverviewFor example, the prefix kilo multiplies by one thousand, so a kilometre is 1,000 metres, and a kilowatt is 1,000 watts. The prefix milli subdivides by a thousand, so a millimetre is one-thousandth of a metre (1000 millimetres in a metre), and a millilitre is one-thousandth of a litre. The ability to apply the same prefixes to any SI unit is one of the key strengths of the SI, since it considerably simplifies the system's learning and use. The most commonly used prefixes include:
Prefixes cannot be combined: for example 10−9 metre should be written as 1 nm, not as 1 mμm. The prefix always takes precedence over any exponentiation; thus "km2" means square kilometre and not kilo–square metre. For example, 3 km2 is equal to 3 000 000 m2 and not to 3000 m2 (nor to 9 000 000 m2). Thus the SI prefixes provide steps of a factor one million instead of one thousand in the case of an exponent 2, of a billion in the case of an exponent 3, etc. As a result large numbers may be needed, even if the prefixes are fully used. Prefixes where the exponent is divisible by three are recommended. Hence "100 m" rather than "1 hm". Notable exceptions include centimetre, hectare (hecto-are), centilitre, cubic decimetre (equivalent to one litre), and decibel (one-tenth of a bel, a measure of sound loudness). The obsolete prefixes myria- and myrio- were dropped before SI was adopted in 1960, probably because they do not fit this pattern, no symbol was available (M, m and μ already being used), and were rarely used anyway. Double prefixes such as micromicrofarads (picofarads), hectokilometres (100 kilometres), and millimicrons or micromillimetres (both nanometres) were also dropped with the introduction of the SI. The kilogram stands out among all SI base units as the only one that has a prefix. It is derived from the mass of an actual object. The gram is defined as 1/1000 of this object's mass. Though in principle legal, most combinations of prefixes with quantities are very rarely used, even in a scientific or engineering context:
PronunciationThe accepted English pronunciation of the initial G of giga was once soft, /ˈdʒaɪgə/ (like gigantic), but now the hard pronunciation, /ˈgɪgə/ (like giggle), is significantly more common. However, both pronunciations are likely to be understood by most English speakers, though the second is likely to be preferred. Use outside SIThe symbol "k" is often used to mean a multiple of a thousand, so one may talk of "a 40K salary" (40,000), or the Y2K problem. Note that in these cases an upper case K is often used, although it should be noted that using an uppercase K is never correct when writing under the rules of the SI. Also, it is often used as a prefix to designate the binary prefix kilo = 210 = 1024. Non-SI units
ComputingMain article: Binary prefix The prefixes k and greater are common in computing, where they are applied to information and storage units like the bit and the byte. Since 210 = 1024, and 103 = 1000, this led to the SI prefix letters being used approximately to denote "binary" prefixes as follows:
However, these prefixes usually retain their powers-of-1000 meanings when used to describe rates of data transmission (bit rates): 10 Mbit/s Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. The problem is compounded by the fact that the units of information (the bit and the byte) are not part of SI. Although it is clearer symbology to use "bit" for the bit and "b" for the byte, "b" is often used for bit and "B" for byte. (In SI, B stands for the bel.) French-speaking countries often use "o" for "octet", a synonym for byte, but this is unacceptable in SI because of the risk of confusion with the zero. Consequently, the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one kilobyte (1 kb) equals 1000 bytes, whereas one kibibyte (1 KiB) equals 210 = 1024 bytes. Likewise mebi (Mi; 220), gibi (Gi; 230), tebi (Ti; 240), pebi (Pi; 250), and exbi (Ei; 260). Although the IEC standard does not mention them, the sequence can be readily extended to zebi (Zi; 270) and yobi (Yi; 280). The adoption of these prefixes has been very limited. † Britain, Ireland, Australia and New Zealand previously used the long scale number name conventions, but have now at least partly switched to the short scale usage. Note in particular that above a million and below a millionth, the same name has different values in the two naming systems, so billion and trillion (for example) become unfortunately potentially ambiguous terms internationally. Using the SI prefixes can circumvent this problem. See number names for the details. Proposed extensionsContinuing backwards in the alphabet, after zetta and yotta, proposals for the next large number include xenta and xona, the latter as an alteration of the Latin nona, and the next small number would also start with an x. Preserving the rule on abbreviating the prefixes (a capital for the large number and a lower-case letter for the small number), even without consensus on the full name the following prefix symbols could be used without ambiguity: X, W, V, x, w, v. (The logically next small prefix symbol, "u", risks confusion with "µ", the symbol for "micro". If that is not a problem then "U" and "u" can be used, but then for the next prefix "T" definitely clashes with "tera".) However, even some official prefixes may not be understood by all readers, let alone extrapolations of them, so giving an explanation is advisable when using them in communication (as opposed to using them in notes for oneself). Another proposal for xenta/xona is novetta, from the Italian nove. This does not have the convenience of backward alphabetic order. See also
ReferencesThis article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. External links
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