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SESQUI-
A combining form (also used adjectively) denoting that three atoms or equivalents of the stuff to the name of which it is prefixed are combined with two of some another element or radical; as, sesquibromide, sesquicarbonate, sesquichloride, sesquioxide.
Sesquidupli- is sometimes, but rarely, used in the samemanner to denote the proportions of two and a half to one, or rather of five to two.
Origin: L, one half more, one and a half.