North-northwest, at 337.5 degrees, sits on the boundary between two mountains of the Chinese geomantic compass: Hai, the branch of the pig, and Ren, a heavenly stem occupying the arc immediately west of due north.
The luopan divides the horizon into twenty-four mountains of fifteen degrees each, built from the twelve earthly branches, eight heavenly stems and the four corner trigrams. The European rose divides it into sixteen or thirty-two points of 22.5 or 11.25 degrees. The two schemes agree on the four cardinals and the four intercardinals and on nothing else, and each of the eight secondary points of the sixteen-point rose falls precisely on a boundary between mountains rather than at the middle of one.
Hai is the last of the twelve branches and Ren stands next to Zi, the branch of due north, so this particular boundary is close to the seam where the cycle of the branches closes and begins again. In the reckoning of hours, Hai is the double hour from nine at night to eleven, immediately before the hour of the rat that contains midnight.
For a navigator the 22.5 degree point is a bearing like any other. For a practitioner reading a luopan it is a line between two readings, and the compass schools treat a building or a grave aligned exactly along it as sitting on nothing. The same arc of sky, divided by two arithmetics, produces a usable direction in one system and a gap in the other.