Ses·qui-  Chem. A combining form (also used adjectively) denoting that three atoms or equivalents of the substance to the name of which it is prefixed are combined with two of some other element or radical; as, sesquibromide, sesquicarbonate, sesquichloride, sesquioxide.
 Note: ☞ Sesquidupli- is sometimes, but rarely, used in the same manner to denote the proportions of two and a half to one, or rather of five to two.

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