Bar-tracery divides the large lights from one another with moulded mullions. Bar-tracery, an important decorative element of Gothic styles, appeared first at Reims and was employed in England around 1240. 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 Reims 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. "Rayonnant" style (c.1230–c.1350) was enabled by the development of bar-tracery in Continental Europe and is named for the radiation of lights around a central point in circular rose windows. "Rayonnant" also deployed mouldings of two different types in tracery, where earlier styles had used moulding of a single size, with different sizes of mullions. The rose windows of Notre-Dame de Paris (c.1270) are typical. The earliest bar tracery designs were made for the aisle windows at Reims Cathedral around 1215. The Reims windows still used the same 'two lancets plus oculus' pattern (as in the Soissons example above), but now the glass panels were held between narrow stone mullions made up of carefully shaped lengths of masonry (fitted together with mortar and metal pins) quite distinct from the wall surrounding them. These mullions were much more slender than the corresponding elements in plate tracery windows and crucially, the previously solid wall areas such as the spandrels could also now be glazed, greatly increasing the amount of light admitted. The cross-section of each mullion or tracery bar was important both for the structural integrity of the window and for the visual effect. As can be seen in Viollet-le-Duc's diagram ("right") there was normally a roll-moulding on both the inside and outside of the windows, which made the mullions appear even more slender than they actually were. The shoulder marked 'B' on the diagram is the glazing slot, into which the metal frame (armature) of the window glass is mounted. Unlike with plate tracery, where each stone had to be individually shaped, the elements of bar tracery could be mass-produced to standard templates in the mason's yard – work that could continue even when it was too cold for lime mortar to set.