Why is “les toilettes” plural in French?

In France you go aux toilettes, plural, even in a flat with a single small room. The usual explanation is habit rather than logic: public facilities come in rows, and the plural stuck to the word. The singular survives with a different meaning altogether — faire sa toilette is washing and getting yourself ready in the morning, not a trip to the bathroom.

The plural is not universal across the French-speaking world: Belgium and Québec commonly say la toilette in the singular. In France, the sentence you actually need is où sont les toilettes, s’il vous plaît ? — and the answer will often send you to le petit coin or, on a sign, to les WC.