The canonical line in
李之仪《
卜算子·
我住长江头》(Li Zhiyi, Northern Song, early 12th c.) is
我住长江头,
君住长江尾 (I live at the Yangtze's head, you at its tail). The reversed form
君住长江头 appears in later circulations and as the symmetrical half when the line is repeated (e.g., in modern song settings where both shores sing alternately).
长江头 = upper Yangtze (Sichuan);
长江尾 = lower Yangtze (Jiangsu/Shanghai).