Alexander Calder gave sculpture two things it had never had: motion, and a household name for it. The mobile — abstract shapes balanced on wires, moving with the air of the room — was invented by this trained mechanical engineer from a family of Philadelphia sculptors, and it happened in Paris.
He arrived in 1926, and his miniature circus of wire and cork, the Cirque Calder, became an avant-garde ticket: Joan Miró, Fernand Léger and Le Corbusier came to watch him animate acrobats by hand on the floor. In October 1930 he visited Piet Mondrian's studio, saw the coloured rectangles pinned to the walls, and turned abstract almost overnight. Marcel Duchamp christened the moving works 'mobiles' in 1931; Jean Arp named the still ones 'stabiles'; and Calder spent four more decades scaling them from tabletop toys to plaza-sized landmarks.