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Smith's Greek and Latin Roots gives the etymology of a few SI prefixes. For example, tera- is from Greek teras ("monster"), deci- from Latin decem, and micro- from Greek mikros ("small").

https://www.nist.gov/pml/owm/metric-si-prefixes lists more SI prefixes, including those that are not shown in Smith's book:

Prefix Abbreviation Value
quetta- Q 1030
ronna- R 1027
yotta- Y 1024
zetta- Z 1021
exa- E 1018
peta- P 1015
zepto- z 10−21
yocto- y 10−24
ronto- r 10−27
quecto- q 10−30

Do these SI prefixes also have etymologies from Greek or Latin? If yes, what Greek or Latin words are they derived from or related to?

2 Answers 2

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From an article on the adoption of the newest prefixes (Q, R, q, r) in 2022:

"The only letters that were not used for other units or other symbols were R and Q," Brown said.

Convention dictates that the larger prefixes end in an A, and the smaller ones in an O.

And "the middle of the words are very, very loosely based on the Greek and Latin for nine and 10," Brown said.

So no, they are not really Greek or Latin words. The new prefixes were designed to be convenient for the intended use, not etymologically sensible.

I have not come across a similar account of the older ones, but the correspondences between the corresponding big and small prefixes (Z/z and Y/y) are so similar to the new ones (R/r, Q/q) that they appear equally artificial. The only ones on your list that look like they might come from something are E and P.

The broad upshot is that these prefixes are to be taken as just a convention, not really versions of Greek or Latin words.

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    seeing as the question also covers Y, Z, E, P, z, & y this answer can be significantly improved by addressing those as well
    – Tristan
    Mar 18 at 8:39
  • @Tristan Fair enough! I added a note on them. I didn't find anything addressing them, but I addressed the matter explicitly. There's certainly room for new answers if someone has something to add.
    – Joonas Ilmavirta
    Mar 18 at 20:07
  • Both "yotta" and "zetta" are also phonetic transcriptions of Greek letter names. The Greek i/I (ι/Ι) is called γιώτα (yota) in Modern Greek and ιώτα (iota, which is where the English word iota comes from) in Ancient Greek, and the Greek z/Z (ζ/Ζ) is ζήτα (zeta of the zeta functions). I suspect that had something to do with their usage here. (and sorry, I realize I am not telling you anything you don't know here Joonas, I just thought it might be worth mentioning).
    – terdon
    Mar 19 at 19:16
  • @terdon That's ok! I know full well that a comment on my post is not always a comment to me. // To my ear the difference between zeta and zetta (and iota and yotta) is substantial, so the connection is less than obvious. But the average American ear will probably disagree with me.
    – Joonas Ilmavirta
    Mar 19 at 20:33
  • That's interesting. Based on your profile, I guess you're a native Finnish speaker and I believe Finnish has a lot of cases where repeated letters change in pronunciation (my friend Janne took great pains to teach me!). As a native speaker of Greek and English, I don't read yotta and yota differently. Maybe, with my English only hat on, I would assume the former had a "short" o (like shot) and the latter a "long" one (like moan or lone), but I hadn't even noticed it before.
    – terdon
    Mar 19 at 20:59
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As you note, the tera- prefix comes from the Greek word for monster.

But it also happens to be quite similar to tetra-, which is of course the Greek-derived prefix for four.

The next step up from tetra- is penta-. Change that slightly and you get the peta- prefix.

As you continue going up, the prefixes continue to be corruptions of Greek/Latin-derived numerical prefixes.

  • sexa-/hexa- becomes exa-
  • septa- becomes zetta-
  • octa- becomes yotta-
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    +1 for giving the correspondence between the prefixes and the Greek/Latin numbers. But it’s not quite accurate to call the SI prefixes “corruptions” of the Greek/Latin numbers — as explained in Joonas Ilmavirta’s answer, they’re deliberate neologisms, chosen to loosely echo the Greek/Latin numbers. Mar 19 at 13:39

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