“God is a circle whose centre is everywhere but whose circumference is nowhere”. Circle means perfection, cyclicity, superiority of the divinity, but also instability and movement. In nature soap bubbles are spherical and internal trees’ rings are circular; the legend tells that Giotto drew a perfect O, while perfection is tangible on Michelangelo’s “Tondo Doni” and Botticelli’s “Vergine col Bambino”. King Arthur’s knights were pairs around a round table, and nowadays people sit in circle to make a decision or watch a show. Bruno Munari selects and describes in this little, extraordinary encyclopedia, several uses of this fascinating and mysterious form, instable and hieratic at the same time.