From Rene Guenon’s The Great Triad:

It is also said that Heaven, which envelops and embraces all things, presents a ‘ventral’ — that is to say inward — face to the Cosmos while the Earth that supports all things presents a ‘ dorsal’ or outward face.  This can be easily grasped by simply looking at the diagram below, in which Heaven and Earth are respectively represented by a concentric circle and a square.

It will be observed that this diagram reproduces the shape of Chinese coins, which also happens originally to have been the shape of certain ritual tablets. The part that the characters are inscribed on — that is, the solid area between the circular outline and the square empty space in the centre — clearly corresponds to the Cosmos comprising the ‘ten thousand beings’. The fact that this area is bounded by two voids is a symbolic expression of the fact that what is not between Heaven and Earth is for that very reason not a part of manifestation.