Cristóbal Balenciaga was one of the few couturiers who could do it all—sketch, drape, cut, sew, finish. His last two decades, the 50s and 60s, were his most brilliant, a period when the mystery of his cut, the volumes he created with a single seam, were like annunciations—generative in their influence. More than 100 iconic garments and hats from those last decades, plus X-rays and digital patterns that reveal the mysteries, make up this show. —L.J.