The people on both banks share the Yangtze's waters - kin by blood.
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From Li Zhiyi's (李之仪, Northern Song) 《卜算子·我住长江头》: 我住长江头,君住长江尾。日日思君不见君,共饮长江水。 (I live at the Yangtze's head, you at its tail. Daily I think of you, never see you - yet we drink the same river.) A short, tender ci lyric beloved for its plain-spoken longing.
usage
Often quoted in cross-strait (两岸) political rhetoric to emphasize shared roots. Also appears in Shanghai/Wuhan/Chongqing sister-city speeches and love-at-a-distance contexts.