<P> As bar tracery opened the way for more complex patterns, masons started applying those same patterns to other surfaces as well as the actual window openings . When used on an otherwise solid walls, such motifs are known as blind tracery, a decorative effect first applied on the west facade of the church of St Nicaise at Reims (1230's). Conversely, tracery was also constructed as openwork screens, which could either match the window tracery behind them (e.g. the Basilica of Saint Urbain, Troyes) or create a visual counterpoint to it, as on the exterior of the west facade of Strasbourg Cathedral . Open tracery in particular was a key feature of the later phases of Rayonnant Gothic . </P> <P> Most 19th - century histories of Gothic architectural style used a series of rather arbitrary categories based on the supposed evolution of the dominant patterns of window tracery . Such teleological models are now regarded as oversimplistic and are generally shunned by art historians, although they live on in the popular literature . In terms of the overall development of Gothic architecture, the crucial development was not so much the use of any particular tracery patterns but the transition from plate - to bar - tracery, which was what made the Rayonnant and subsequent styles possible . </P> <P> As the complexity of tracery increased, so did the need for masons to draw out their designs in advance, either as a way of experimenting with patterns or as a way of communicating their designs to other craftsmen or to their patrons . Because of the cost and size limitations of parchment sheets, such designs would normally be drawn by incising onto a whitewashed board or a conveniently placed section of flat wall . In the latter case, the wall would be prepared with a thin layer of plaster, which would show the design more clearly . </P> <P> A number of churches and cathedrals still show the faint remains of these tracings (or épures as they are known in France), from where the mason's compass points scratched through the plaster and into the masonry below . (Examples include some experimental 14th century window tracery patterns at the eastern end of the south wall inside the Galillee porch of Ely Cathedral, or the extensive series of tracings on the flat aisle roofs of Clermont - Ferrand Cathedral .) A number of major building sites (including Westminster Abbey, Wells Cathedral and York Minster) originally had dedicated tracery chambers, where the architects could prepare their designs in relative comfort . The availability of a large flat floor surface meant that designs could be drawn life - size and the individual elements of bar tracery laid out on the plan to test their goodness of fit, before hoisting them up the scaffolding for installation in the actual window openings . This also meant that masons could carry on working through the winter season, when building work would normally grind to a halt . </P>

Ornamental architectural openwork in the upper part of some gothic window