The Canterbury cap is a square cloth hat with sharp corners found in the Anglican communion, similar to the Counter-Reformation's biretta, the notable exception being that a Canterbury cap has four ridges, compared to the biretta's three. It is also soft and foldable, "Constructed to fold flat when not in use..." whereas the biretta is rigid. The Canterbury cap is the medieval birettum, descended from the ancient pileus headcovering. It is sometimes called the "catercap." The Tudor bonnet is also a similar academic cap worn by a person who holds a doctorate.