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I learned that the word "dyadic", a word opposed to monadic, is written with a y.

My etymology sources indicates it comes from "Dyad", which means "pair", "couple", "double". However, every other word I know with a "di-" prefix meaning "two" is written with an i. (Dioxyde, dipterous, dioptre, digram, ... The "bi-" prefix is now much more prevalent but every other case of "di-" I know uses an i)

Even in ancient Greek, there's a difference between the "δι-" prefix and the word "δυάς", they don't use the same letter, although they both come from the Greek word for "two".

  • Why is there a difference in spelling between the Greek word for two and the "di-" prefix?

  • Why does "dyadic" seem to be the only word that kept the y?

Note that this seems to date from ancient Greek, so as far as I know it also happens in every language that got these words from Greek, not just English.

Note2: I should have specified it, but of course I'm talking about words where the "dy-" prefix means "two", so this is not about words like "dynamic" or "dyssentry".

  • There is also dyarchy and dyotheism, but you are right that there don't seem to be many such words. – Xavier Aug 26 '19 at 23:47
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    "as far as I know it also happens in every language that got these words from Greek" this is not true at least for Italian, Spanish and Serbian – fqq Aug 27 '19 at 9:18
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    @fqq My bad, I saw it appearing in French and English. That's why I said "as far as I know"! I assume it was corrected in order to make spelling consistent (for Spanish at least). – Teleporting Goat Aug 28 '19 at 15:38
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The prefix δυ- is from δύω “two” < IE *duō. The prefix δι- is from δίς “twice” < IE *dwi- (the /w/ is lost in Greek). Both are common in Greek.

By the way: “division” is from Latin, not Greek.

There are lots of English words beginning with dyo- and dy-. I suggest you flick through any English dictionary.

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  • I did look up in a dictionary, but most words are either alternate spelling of more commons "di-" words (like diarchy or dioptre), or words with unrelated etymology, mostly using the "dys-" prefix. I edited my question, I should have been clearer. Also, "dyadic" seems to be the only one that can't be spelled with "di-". – Teleporting Goat Aug 27 '19 at 7:59
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    @TeleportingGoat "Lots" might be an overstatement, but it shows up mostly in technical terms formed from Greek roots. The Christian belief that Christ had two wills, for example, is "dyothelism", a signal that differentiates two different states is "dyotic", a chemical reaction that switches two single bonds at the same time is "dyotropic", a hypothetical particle with both an electric and magnetic charge is a "dyon". It's worth noting that these are mostly modern formations, not ancient ones. – Draconis Aug 28 '19 at 18:45
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Dyad is a commonly-used musicological term for a two note chords. Pedal steel guitars use this harmonic structure primarily.

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    While true, dyad is just the noun form of dyadic. – user91988 Aug 28 '19 at 16:23
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    Like only_pro said, while this is absolutely true, the question excludes this word specifically: "My etymology sources indicates it comes from "Dyad", which means "pair", "couple", "double". However, every other word I know…" (Emphasis mine.) – Draconis Aug 28 '19 at 18:39
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    ah yes, pedal steel guitar morphology for harmonic declension – Carly Aug 29 '19 at 17:50

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