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pair; couple; brace
一つがいの鳥がいるよ。
There's a pair of birds.
池に一つがいの白鳥が泳いでいた。
A pair of swans was swimming in the pond.
一つがいの鶴が毎年この庭に巣を作りに来るそうだ。
A pair of cranes apparently comes to build a nest in this garden every year.