白头偕老 (bái tóu xié lǎo) — Grow Old Together · White Hair Side by Side · Lifelong Companionship

The Ministry's idiom dictionary builds the phrase from two older pieces, not from a single classical line that already reads 白头偕老. 白头 — white hair — is taken from the Han Yuefu poem 《白头吟》, whose wish-line is 愿得一心人，白头不相离: may I find one wholehearted person, and stay with white hair and never part. That poem is a rejection poem, not a wedding toast; what later speech kept is the image of white hair as the proof that two people stayed. 偕老 — grow old together — is taken from 《诗经·郑风·女曰鸡鸣》, where a couple at ease over wine says 宜言饮酒，与子偕老. The Book of Songs line is domestic and forward-looking; it does not yet need the white-hair marker.

Once the two halves fuse, the living four-character form does a job neither half does alone. 白头 supplies a visible check you can take without opening a calendar. 偕 supplies the grammar that needs two people: 偕老 is not something one person can predicate of themselves the way 长情 can. Moedict glosses the compound as a married couple living together until old age — lifelong companions — and cites Ming drama already using the fixed form as ordinary speech: 我与你母亲白头偕老.

Against the near neighbour on the wedding banner, the split is sharp. 百年偕老 asks that the years run out side by side. 白头偕老 keeps 偕老 and adds the body as evidence: the wish is not only that time passed together, but that you can see it did.

Today the phrase is first a wedding congratulation. The Ministry's usage notes put it squarely on 恩爱终生 speech — guests blessing a new couple, lovers promising each other, families hoping the marriage holds to old age — and lists 白头到老 as the everyday variant. Calligraphy gifts and red banners still reach for it because four characters name a complete arc without decorating it: no dragon, no moon, no counted century, only the hair and the togetherness.

The phrase also survives the years after the ceremony. At a long anniversary the same four characters change tense without changing wording: what was asked for at the wedding can be pointed at in the room. That double use is why it sits beside 百年好合 on the traditional banner set without being redundant — one asks about the quality of the union across a counted span; this one asks whether both people are still there when age has written itself on their heads.