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. The technical aspects of the windows at Reims clearly fascinated Villard de Honnecourt, who visited the construction site, probably in the 1220s, and made a detailed sketch of the various templates, using a key to show how they fitted into the different parts of the window (the templates are in the lower half of folio 32 recto; the symbols besides the templates match similar