It has been shown how the grouping of lancet-shaped windows and the piercing of the space above them, under the arched dripstone, had produced “plate tracery.” As the piercings became larger, narrow and irregularly shaped surfaces of stone were left. These were pierced, and the intervening piers of stone came to be shaped like the mullions; in fact, became a continuation of the mullions. This development, which was reached before the middle of the 13th century, is called “bar tracery.” At first, this bar tracery was plain; then “cusps” (Pl. 40, Fig. 5), or projecting points, were introduced on the inner edge of the mullions, and added greatly to the rich effects. The earliest Decorated windows have tracery on a purely geometric basis. Exeter Cathedral is considered the best typical example of the early part of this style, and the existing windows were constructed at the end of the 13th century. The Chapter-houses at York and Southwell are other rich examples. Windows with flowing tracery are, in general, later than those with geometrical patterns, though they are sometimes contemporaneous in the same building. No rule is followed in the form of the arch over windows in this style. Some are very obtuse, others very acute, and the ogee, or double-curved arch, is not uncommon. Square-headed windows are very common in this style, in many parts of the country, especially in Leicestershire and Oxfordshire. This form of window was so very convenient that its use was never discontinued, though it was more commonly used in houses and castles than in churches, and windows with a flat curved top are frequently used. Circular or “Rose” windows in churches and cathedrals are also a fine feature of this style. Notable among these are the windows at the end of the south transept in Lincoln and Westminster.