Editing Metric prefix (section)

== Non-standard prefixes ==
{{see also|Unit prefix#Unofficial prefixes}}
[[File:Myriameterstein36RüdesheimRhein.JPG|thumb|Distance marker on the [[Rhine]] at [[Rüdesheim am Rhein|Rüdesheim]]: 36 (XXXVI) myriametres from [[Basel]]. The stated distance is 360 km; the [[decimal mark]] in [[Germany]] is a comma.]]

=== <span class="anchor" id="myria"></span><span class="anchor" id="myrio"></span><span class="anchor" id="hebdo"></span><span class="anchor" id="micri"></span><span class="anchor" id="double"></span><span class="anchor" id="demi"></span><span class="anchor" id="Obsolete"></span>Obsolete metric prefixes ===
Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.<ref>{{Cite web
| title = H.R. 596, An Act to authorize the use of the metric system of weights and measures 
| publisher = 29th Congress of the United States, Session 1
| date = 13 May 1866
| url = http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html
| url-status = dead
| archive-url = https://web.archive.org/web/20150705015307/http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html
| archive-date = 5 July 2015
}}</ref><ref name="Brewster_1830">{{Cite book
| first = David
| last = Brewster
| date = 1830
| title = The Edinburgh Encyclopædia
| volume = 12
| location = Edinburgh, UK
| publisher = William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J.M. Richardson 
| page = 494
| url = https://books.google.com/books?id=0bIkTUZAbxcC
| access-date = 9 October 2015
}}</ref><ref name="Brewster_1832">{{Cite book
| first = David
| last = Brewster
| date = 1832
| title = The Edinburgh Encyclopaedia
| volume = 12
| edition = 1st American
| publisher = Joseph and Edward Parker
| url = https://books.google.com/books?id=17RGAQAAIAAJ&pg=PA572
| access-date = 9 October 2015
}}</ref> The decimal prefix for ten thousand, ''[[myria-]]'' (sometimes spelt ''[[myrio-]]''), and the early [[#Binary prefixes|binary prefixes]] ''double-'' (2×) and ''demi-'' ({{sfrac|1|2}}×) were parts of the original metric system adopted by France in 1795,<ref name="fr">{{Cite book
| section = La loi du 18 Germinal an 3
| quote = Décision de tracer le mètre, unité fondamentale, sur une règle de platine. Nomenclature des « mesures républicaines ». Reprise de la triangulation
| trans-quote = ''The Law of 18 Germinal &#91;month&#93;, Year 3: Decision to draw the fundamental unit metre on a platinum ruler. Nomenclature of "republican measures". Resumption of the triangulation''
| language = fr
| title = L'histoire du mètre
| trans-title = The History of the Metre
| via = histoire.du.metre.free.fr
| section-url = http://histoire.du.metre.free.fr/fr/Pages/Sommaire/06.htm
| access-date = 2015-10-12
| url-status = live
| archive-url = https://web.archive.org/web/20221126164814/http://histoire.du.metre.free.fr/fr/Pages/Sommaire/06.htm
| archive-date = 2022-11-26
}}</ref>{{efn|
	"Art. 8. Dans les poids et mesures de capacité, chacune des mesures décimales de ces deux genres aura son double et sa moitié, afin de donner à la vente des divers objets toute la commodité que l'on peut désirer. Il y aura donc le double-litre et le demi-litre, le double-hectogramme et le demi-hectogramme, et ainsi des autres. 
	| trans-quote =
	Art. 8. In the weights and measures of capacity, each of the decimal measures of these two kinds will have its double and its half, in order to give to the sale of the various articles all the convenience that one can desire. There will therefore be the double-litre and the half-litre, the double-hectogram and the half-hectogram, and so on."<ref name="fr"/>
}}
but were not retained when the SI prefixes were internationally adopted by the 11th [[Conférence générale des poids et mesures|CGPM conference]] in 1960.

Other metric prefixes used historically include [[hebdo-]] ({{10^|7}}) and [[micri-]] ({{10^|−14}}).

=== <span class="anchor" id="dimi"></span><span class="anchor" id="Double prefix"></span> Double prefixes ===
Double prefixes have been used in the past, such as ''micromillimetres'' or ''millimicrons'' (now [[nanometre]]s), ''micromicrofarads'' (μμF; now [[picofarad]]s, pF), ''kilomegatonnes'' (now [[gigatonne]]s), ''hectokilometres'' (now 100&nbsp;[[kilometre]]s) and the derived adjective ''hectokilometric'' (typically used for qualifying the fuel consumption measures).<ref name="Rowlett_2008_Millimicro">{{Cite dictionary
| first = Russ
| last = Rowlett
| year = 2008
| orig-year = 2000
| title = millimicro-
| dictionary = How Many? A dictionary of units of measurement
| publisher = [[University of North Carolina at Chapel Hill]]
| access-date = 29 August 2016
| url = http://www.unc.edu/~rowlett/units/dictM.html
| url-status = live
| archive-url = https://web.archive.org/web/20160829225351/https://www.unc.edu/~rowlett/units/dictM.html
| archive-date = 29 August 2016
}}</ref> These are not compatible with the SI.

Other obsolete double prefixes included "decimilli-" ({{10^|−4}}), which was contracted to "dimi-"<ref>{{Cite book
| first = Maurice
| last = Danloux-Dumesnils
| date = 1969
| title = The Metric System: A critical study of its principles and practice
| publisher = The Athlone Press
| page = 34
| isbn = 9780485120134
| url = https://books.google.com/books?id=ElHAAAAAIAAJ
| access-date = 9 October 2015
}} (a translation of the French original {{lang|fr|Esprit et bon usage du système métrique}}, 1965 <!-- or ''Étude critique du système métrique'', 1962 -->)</ref> and standardised in France up to 1961.

There are no more letters of the Latin alphabet available for new prefixes (all the unused letters are already used for units). As such, Richard J.C. Brown (who proposed the prefixes adopted for {{10^|±27}} and {{10^|±30}}) has proposed a reintroduction of compound prefixes (e.g. ''kiloquetta-'' for {{10^|33}}) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always be ''quetta-'' or ''quecto-''. This usage has not been approved by the BIPM.<ref>{{Cite journal
| first = Richard J.C.
| last = Brown
| date = 27 April 2022
| title = Reply to "Facing a shortage of the Latin letters for the prospective new SI symbols: Alternative proposal for the new SI prefixes"
| journal = Accreditation and Quality Assurance
| volume = 27
| issue = 3
| pages = 143–144
| doi = 10.1007/s00769-022-01499-7
| s2cid = 248397680
}}</ref><ref>{{Cite journal
| last1 = Brown
| first1 = Richard J.C.
| year = 2019
| title = Considerations on compound SI prefixes
| journal = Measurement
| volume = 140
| pages = 237–239
| doi = 10.1016/j.measurement.2019.04.024
| bibcode = 2019Meas..140..237B
| s2cid = 146092009
}}</ref>