Sir Cumference is a series of children's educational books about math by Cindy Neuschwander and Wayne Geehan.
[1] Most of the characters of the book are named after math terms, such as Sir Cumference (circumference).
He has a wife called Lady Di of Ameter and a son named Radius.
He has a friend named Vertex in Sir Cumference and the Sword in the Cone, and plays an important role in both Sir Cumference and the Dragon of Pi and The Sword in the Cone first by turning his father to a dragon and back, and later assisting Vertex in becoming King.
Sir Cumference and Radius agree Vertex should be the heir to the throne.