Joan Miró was a small, fastidious, taciturn Catalan. Alexander Calder was a big, rumpled, gregarious American. At first glance, they would appear to hail from distant planets. Yet once they met in Paris in 1928, they enjoyed an unusually close and mutually beneficial friendship that lasted until Calder’s death in 1976. With other artistic pairs, like Pissarro and Cézanne or Picasso and Braque, competitiveness ignited and acrimony at times soured the creative ferment. But Miró and Calder unfailingly championed and nourished each other’s work. A principle that Calder applied to his art could also describe their relationship: “Disparity in form, color, size, weight, motion, is what makes a composition.”