A variant form of copartnership; a joint partnership with shared authority and responsibility.
From co- + partnery (partner + -ery suffix, an older form of -ery denoting an association or practice). This is a less common variant of copartnership.
Copartnery is almost extinct—it lost out to copartnership in standard usage, showing how even near-synonyms made from the same roots can compete until one survives and the other becomes archaic.
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