Bi, the Wall of the Black Tortoise

Bi, the Wall, is the fourteenth of the twenty-eight xiu and the last mansion of the northern palace, the Black Tortoise, one of two mansions romanised Bi, this one meaning wall. It is identified with the remaining stars of the Great Square of Pegasus and Andromeda, and it was anciently associated with libraries and writing. In the later astrological tradition its animal totem is the Porcupine. As one of the twenty-eight lunar mansions it closes the seven of the northern palace. These are the conventions of the Chinese astronomical and calendrical tradition, in which the twenty-eight xiu order the heavens into four palaces; with the Wall the Black Tortoise of winter gives way to the White Tiger of the west and autumn.

Bi the Wall closing the northern palace shares its transliterated name with the nineteenth mansion, Bi the Net, in the western palace, another instance of the accidental romanisation collisions that recur across the twenty-eight xiu; the two are entirely different Chinese characters, contexts and asterisms, and the coincidence is worth flagging precisely because it has misled more than one modern popular account that treats the repeated spelling as evidence of a deliberate structural pairing rather than an artefact of transliteration.