The rose window
Radial bar tracery in a circle: spokes, cusped heads and foiled oculi, or the simpler Romanesque wheel.
The rules it illustrates
The rose: a window turned about its centre. Design a rose by dividing the circle into equal sectors; fill each with the same traceried unit (a light with a cusped pointed head, an oculus above) radiating from a central hub, so that the whole is generated from one sector by rotation.
Where it appears in the book
In the chapters Romanesque and Gothic.