# The origin of femto, atto and zepto SI prefixes

Do you know why the SI prefixes: femto, atto, zepto have been accepted by Scientific Community, if this triad of metric units, is neither greek nor latin?

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This isn't really a physics question, is it? By the way, the prefixes don't have to come from Greek or Latin. They were adopted by resolution of the Conférence Générale des Poids et Mesures, the SI decision-making body. –  EnergyNumbers Sep 1 '12 at 17:17
@EnergyNumbers: So French then? –  C.R. Sep 1 '12 at 17:52
@ Karsus Ren, It is not french either. femto comes from the danish femten =15 atto comes from the danish atten =18 Zepto ...no idea –  Shaktyai Sep 1 '12 at 18:05
According to Wikipedia "it (zepto) comes from the French sept or Latin septem, meaning seven, since it is equal to $1000^{−7}$". –  John Rennie Sep 2 '12 at 6:29
@EnergyNumbers Questions about units and metrology have historically been considered on topic here. Sure, they're not really physics-specific, but physics seems to concern itself with the study of units more than other sciences, in a way... in any case while you do have a valid point about this not being a physics question, I think this might be okay here. I'm waiting to see what the community thinks. –  David Z Sep 2 '12 at 19:57

From http://www.exa.com.au/metric/ a rewrite of the NIST Constants, Units & Uncertainty home page

Prefixes ranging from micro to mega were first introduced in 1874 by BAAS as part of their CGS system. Later, 12 prefixes ranging from pico to tera were defined as part of the International System of Units - SI, which was adopted in 1960. SI is maintained by BIPM under exclusive supervision of CIPM and resolutions made by CGPM. Further 8 prefixes were added to SI in years 1964 (femto, atto), 1975 (peta, exa) and 1991 (zetta, zepto, yotta, yocto).

exa alteration of hexa from Greek hex meaning "six" (the sixth power of 10^3)

peta alteration of penta from Greek pente meaning "five" (the fifth power of 10^3)

tera from Greek teras meaning "monster"

giga from Greek gigas meaning "giant"

mega from Greek megas meaning "great"

kilo from Greek khilioi meaning "thousand"

hecto French, alteration of Greek hekaton meaning "hundred"

deca from Greek deka meaning "ten"

deci from Latin decimus meaning tenth, from decem meaning "ten"

centi from Latin centi-, from centum meaning "hundred"

milli from Latin mille meaning "thousand"

micro from Greek mikros meaning "small"

nano from Greek nannos meaning "dwarf"

pico from Spanish pico meaning "small quantity"

femto from Danish, or Norwegian word femten meaning "fifteen"

atto from Danish, or Norwegian word atten meaning "eighteen"

The names zepto and zetta are derived from septo, from Latin septem which means seven (the seventh power of 10^3) and the letter 'z' is substituted for the letter 's' to avoid the duplicate use of the letter 's' as a symbol in SI. The names yocto and yotta are derived from Latin octo which means eight (the eighth power of 10^3); the letter 'y' is added to avoid the use of the letter 'o' as a symbol because it may be confused with the number zero. The CGPM has decided to name the prefixes, starting with the seventh power of 10^3, with the letters of the Latin alphabet, but starting from the end. Therefore the choice of letters 'z' and 'y'. The initial letter 'h' of the word hexa in standard French is silent, so it was removed in order to simplify things.

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Due to already mentioned shift in this paradigm (and doubt about syntax), it is not so easy to explain/justify to the kids (students) well established prefixes ¿isn't it?... –  paritto6 Dec 1 '12 at 14:34
Additional references: [1] (bipm.org/en/si/si_brochure/chapter3/prefixes.html) [2] (en.wikipedia.org/wiki/… meetings) –  paritto6 Dec 1 '12 at 14:38
I guess the h in t (h)exa has been removed to avoid ambiguity with hecto, at least in abbreviations –  Frédéric Grosshans Apr 25 '13 at 18:31