In card games like bridge, a holding of exactly two cards in a suit, or a town or place name containing 'double' in it.
From 'double' plus the suffix '-ton,' which is borrowed from 'singleton' (a card-game term for holding only one card). This specialized vocabulary developed in bridge and other trick-taking card games in the early 20th century.
Bridge players developed an entire vocabulary around how many cards you hold—singleton, doubleton, tripleton—because knowing whether partner has two or one card in a suit can determine whether you win or lose the entire game.
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