Divide a circle in half through the center, then divide it again and again into eight 'slices' — from each point where the dividing lines intersect with the circle's edge, draw lines to intersect with the other intersection points. This armature of the circle provides useful structure when creating compositions within a circular space.
Not all paintings are rectangles. When creating a circular composition, called a tondo, artists through history have used the armature of the circle, or the 'medallion' to craft a harmonious composition. Now, you can wear this ancient and hypnotizingly beautiful grid on one of the most comfortable tees I've ever found.