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pair; couple; brace
ひとつがいの鳩がいた。
There was a pair of pigeons.
ひとつがいのカモが池で泳いでいる。
A pair of ducks is swimming in the pond.
ひとつがいのツバメが軒下に巣を作ったのを見つけた。
I found that a pair of swallows had built a nest under the eaves.