Si

Mountain 12 of the twenty-four mountains (luopan) is Si, the Earthly Branch also carried in the Chinese zodiac as the Snake, occupying position SE3, southeast sector, 142.5 to 157.5 degrees. Si is a yin branch whose fixed agent is Fire, and it is a growth branch: within it the sexagenary system stores three hidden stems, Bing (yang Fire) as its principal qi, with Wu (yang Earth) and Geng (yang Metal) carried inside it. In the trine groupings that gather the twelve branches into four three-branch sets, one for each of the four elemental phases, Si functions as the branch that opens the Metal trine.

Twelve of the ring's twenty-four positions are branches rather than stems or trigrams, and they are the only ring of the three that also carries a fixed two-hour span of the traditional Chinese day, Si's own double-hour, and a solar month, so a branch mountain names a direction, a time of day and a season in one and the same character, a triple duty neither the stem ring nor the trigram positions carry. Because ten stems and twelve branches do not share a common factor larger than two, a given stem does not return to a given branch until the combination has cycled sixty times, the sexagenary count (jiazi) that names years, months, days and hours across the traditional Chinese calendar; a mountain's own stem or branch name is therefore also, in a different context entirely, a recurring building block of that sixty-term cycle.

Si sits directly opposite mountain 24, Hai, 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 Si and the front opened toward Hai, 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 Si 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.

As a growth branch, Si is read in Flying Star and Four Pillars applications not only for the direction it names but for the hidden stems it carries within it, since a branch's concealed qi can be drawn on by a chart or a siting even when the branch's visible element alone would not obviously support the reading, which is why storehouse and growth branches are treated as carrying more than their single surface element suggests. 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.