represents the double symbol of Heaven and Earth. Its oval and convex shell represents the vault of heaven, while the square plaque on its underside symbolises the surface of the Earth. The tortoise is the emblem of longevity and perfection, and is generally represented with a coral branch in its mouth and a box containing the sacred book of "Lac Thu" placed under its back. This book serves as a reminder of the invention of a diagram representing the division of the universe into male and female principles; this diagram as made by Emperor Dai Vu (2205-2197 B.C.), and was inspired by his study of the sacred tortoise's shell.