Prêt-à-porter is the French name for ready-to-wear, meaning clothes produced in standard sizes rather than cut for one client, and it is what most catwalk collections actually sell. Merriam-Webster dates the borrowed phrase in English to 1959, when Paris was discovering that not every customer wanted three fittings for a skirt.
The turning point came on 26 September 1966, when Yves Saint Laurent opened his Saint Laurent Rive Gauche boutique at 21 rue de Tournon, on the Left Bank. He was the first couturier to open a ready-to-wear shop under his own name, and he refused to treat it as couture’s cheaper cousin: the collection was designed separately, and its prototypes received the same care as his made-to-measure work. The snobbery about clothes bought straight off the rail has never quite recovered, and the major fashion weeks now revolve around exactly these collections.