 After 1220, master builders in England had begun to treat the window openings as a series of openings divided by thin stone bars, while before 1230 the apse chapels of Rheims Cathedral were decorated with bar-tracery with cusped circles (with bars radiating from the centre).Bar-tracery became common after c.1240, with increasing complexity and decreasing weight.The lines of the mullions continued beyond the tops of the window lights and subdivided the open spandrels above the lights into a variety of decorative shapes.