From Middle English tweyne, tweien, twaine, from Old English twēġen ("two"), from Proto-West Germanic *twai-, from Proto-Germanic *twai, from Proto-Indo-European *dwóh₁. Cognate with Saterland Frisian twäin, Low German twene, German zween, Swedish tvenne . More at two.
The word outlasted the breakdown of gender in Middle English and survived as a secondary form of two, then especially in the cases where the numeral follows a noun. Its continuation into modern times was aided by its use in KJV, the Marriage Service, in poetry (where it is commonly used as a rhyme word), and in oral use where it is necessary to be clear that two and not to or too is meant.
From Middle English twaynen, from twayne (see Etymology 1 above).
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