Mountain 14 of the twenty-four mountains (luopan) is Wu, the Earthly Branch also carried in the Chinese zodiac as the Horse, occupying position S2, due south, 172.5 to 187.5 degrees. Wu is a yang branch whose fixed agent is Fire, and it is a pure cardinal branch: within it the sexagenary system stores two hidden stems, Ding (yin Fire) as its principal qi, with Ji (yin Earth) held within, so its blazing yang face conceals a yin heart. In the trine groupings that gather the twelve branches into four three-branch sets, one for each of the four elemental phases, Wu functions as the cardinal Fire branch, anchoring the Fire trine it forms with Yin and Xu.
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, Wu'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.