|Erik Østergaard - Bit(s) and Byte(s) Article: The peta- principle / Bit(s) og byte(s) artikel: The peta- principle|
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Since shortly after the French Revolution of 1789, scientists -- and eventually computer scientists -- have found it convenient to refer to large quantities of various units of measure with verbal prefixes and letter symbols. Thus kilo- (symbol K) and mega- (M) stand for 10**3 and 10**6, thousands and millions. After World War II, this pair of terms was extended, three zeroes at a time, to giga- (G) and tera- (T), standing for 10**9 and 10**12, billions and trillions (usage here and below is American). In 1975, the world arbiter of the metric system, the General Conference of Weights and Measures (CGPM), based at Sevres near Paris, agreed to add two more terms to the ascending series: peta- (P) and exa- (E) for 10**15 and 10**18, quadrillions and quintillions.
The creation of these newest terms is interesting. The older prefixes, kilo-, mega-, giga-, and tera-, are generally understood to be derived from the ancient Greek words for "thousand", "large", "giant", and "monster", respectively. But peta- departs from the traditional pattern to the extent that there is no Greek (or any other) word to explain it in its present form.
Considering the context, however, (preceding exa-) it is the Greek prefix for "five", penta-, minus the letter "n". The reduction of five letters to four makes it similar in this respect to the existing prefixes. On the other hand, exa- has been reduced to three letters by dropping the "h" from hexa-, the Greek prefix for "six", possibly because the "h" would be silent in standard French.
According to a CGPM report, prefixes meaning "five" and "six" are used because 10**15 and 10**18 are fifth and sixth in the ascending series 10**3, 10**6, 10**9, and 10**12.
(If tera- for 10**12 were taken to mean the Greek prefix for "four", te(t)ra-, minus the second letter "t", that would be an additional reason for its being followed by pe(n)ta- as the prefix for "five". However, the acceptance of tera- as derived from "teras", the Greek word for "monster", seems to be universal.)
The use of Greek words for magnitudes through 10**12 turns out to be awkward in the late twentieth century, since it is difficult to extend the series easily. (After all, what could be bigger than giants and monsters?) Yet it would be even more awkward to throw away the established size-words and start from scratch. Therefore, a backward switch in midstream from a Greek-conceptual to the original Greek-numerical basis (using kilo- for "thousand") is announced as the "expedient" thing to do, simultaneously preserving tradition and leaving the way open to further expansion.
Since 1975, though, and so quietly that current editions of the Encyclopedia Brittanica and World Almanac do not recognize their advent, prefixes with corresponding symbols have materialized for 10**21 and 10**24: zetta- (Z) and yotta- (Y), denoting sextillions and septillions. With these it is easy to see yet another change of direction. While still disyllabic, the names are now semi-artificial (echoing Greek zeta and iota) and symbol-driven, and the series is now based on the Latin alphabet, starting with the last letter and moving backwards, with a long way to go before reaching A. Could this be the last terminological contortion?
Sources: Various books, the Internet, and various encyclopedias.
Kilder: Forskellige bøger, internettet og forskellige leksikoner.