In 1920 Vantongerloo moved to Menton in southern France and there developed a theory of colour in which the three primary colours favoured by De Stijl artists were exchanged for the seven main colours of the spectrum. In addition, mathematical formulae and equations played an increasingly important part in his work. His sculptures still consisted of rectangular forms, but their construction was more open. A similar development took place in his paintings, where some of the rectangular blocks produced by the irregular grid were left white. At this time Vantongerloo began designing interiors, furniture and ceramics, as well as utopian architectural projects (villas, airports and bridges) that were never realized.