The northern palace holds seven lodges, running Dipper, Ox, Girl, Emptiness, Rooftop, Encampment and Wall, from Sagittarius across Capricornus and Aquarius into Pegasus. It is the quarter that culminates in the evening in winter, and it carries the correspondences of winter, the phase Water, the colour black and the direction north.
Its emblem is the only one of the four that is not a single creature. The dragon, the tiger and the bird are each one animal; the north is a tortoise with a snake wound around it, the two usually shown with heads turned toward each other. Explanations offered for the pairing include the reading of the tortoise as female and the snake as male, so that the emblem shows a union rather than a beast, and readings of the shell as the vault of heaven over the flat earth. None is decisive and the pairing is old enough that its origin is not recoverable.
The name is also misleading in English. Xuanwu is more accurately the dark warrior or the mysterious warrior than the black tortoise, and the martial sense of the second element is not incidental.
The tortoise carried another association into the scheme. Shang divination was performed by applying heat to prepared turtle plastrons and ox scapulae and reading the cracks, and the oldest surviving Chinese writing is the record of those consultations inscribed on them. An animal whose shell was the instrument for asking about the future was already the right animal to set at the dark quarter of the sky.