Mao sits directly opposite mountain 20, You, twelve positions and a hundred and eighty degrees around the ring: in compass school feng shui a building or a grave is described by this sitting-and-facing pair, the back set toward Mao and the front opened toward You, and Stephen Skinner's account of the luopan treats the correct reading of a sitting-facing pair as the compass's most basic function, prior to any of the finer rings layered around it. The twenty-four mountain ring itself interleaves three different kinds of position at fifteen degrees each, twelve Earthly Branches, eight Heavenly Stems and four corner trigrams, and Mao belongs to the branch layer, the oldest and most complete of the three, present at every one of the twelve thirty-degree sectors the ring divides the circle into.
Mao's own due-cardinal position is also, in the Later Heaven Bagua arrangement recorded separately in this atlas, associated with one of the four cardinal trigrams, Zhen (Wood), though the ring inscribes only the branch glyph at that point rather than duplicating the trigram beside it. Compass school practice treats the fifteen-degree seam between one mountain and its neighbour as a genuine hazard rather than a rounding error: a sitting or facing line falling exactly on that boundary, called an empty or straddling line, is read as drawing on the qi of both adjacent mountains at once rather than cleanly on one, and Stephen Skinner's account of the luopan treats avoiding a seam-line reading as one of the instrument's basic disciplines, checked before any of the finer rings above the twenty-four mountains are consulted at all.