Once the four animals held the four quarters of the sky they were used to hold the four sides of anything else. Han period tombs put the dragon on the eastern wall, the tiger on the western, the bird on the southern and the tortoise on the northern, so that the burial chamber reproduced the heavens and the body inside it lay at the centre of a properly oriented world. The same four appear on the round eaves tiles of Han buildings, one animal to each of the four directions of a compound.
In siting practice the scheme becomes a rule about ground. Facing out from a building, the azure dragon is the left hand side, which is the east, and the white tiger the right; the vermilion bird is the open ground in front and the dark tortoise the higher ground behind. A good site has the dragon side slightly higher and longer than the tiger side, and manuals discuss what to do when it is not.
Two distinctions are worth keeping. The azure dragon is not the five clawed imperial dragon of robes and thrones, which is a symbol of the emperor's person and a much later regulated emblem; the two are constantly conflated in popular writing. And the scheme travelled. The same four guardians appear in Korean tomb painting, in the painted chambers of early Japanese tombs, and in the gate names of the Japanese capital, where the southern gate of the palace enclosure carried the vermilion bird's name and the eastern quarter the dragon's.