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discarded tiles; discards
河を見れば相手の手が読める。
You can read your opponent's hand by looking at the discards.
河の字牌がとても多いね。
There are so many honor tiles in the discards.
あの人の河を見ると混一色を狙っているようだ。
Looking at their discards, it seems they're going for a flush.