连理

Comes from the image of two trees whose branches and trunks fuse together - 连理枝 ('intertwined branches') symbolizes inseparable love. Bai Juyi's 《长恨歌》 (Song of Everlasting Sorrow) immortalized the line: '在天愿作比翼鸟，在地愿为连理枝' ('In the sky, may we be paired-wing birds; on earth, intertwined branches').

usage

Almost always in the wedding-blessing phrase 结为连理 ('to be joined as intertwined branches' = to marry) or the bouquet term 连理枝. Literary, romantic register.