Wu is the fifth of the ten Heavenly Stems (tiangan), distinct from the Earthly Branch of the same romanisation, the branch Wu of the Horse. Its element is Earth and its polarity is yang, so this stem is glossed as yang Earth, likened in tradition to a mountain or a wall, solid, high, and enduring. As one of the ten stems it combines with the twelve Earthly Branches to form the sexagenary cycle of sixty that numbers years, months, days, and double-hours in the traditional calendar. Standing at the middle of the ten, it carries the yang aspect of the central Earth phase. These are the conventions of the Chinese calendrical and cosmological tradition, used in the almanac and in the four-pillars method of reading a fate from the stems and branches of a birth.
Wu is the fifth Heavenly Stem, the first of the pair carrying the Earth phase, and as an odd-numbered stem it takes the Yang polarity, traditionally likened to a mountain or high plateau, firm and unmoving, against Ji's Yin Earth, likened to level cultivated soil. Wu sits at the structural centre of the ten-stem cycle, a position that echoes Earth's own frequent placement at the centre of the five phases rather than on the points of a single ring. Within the sixty-term sexagenary cycle, Wu combines only with Yang-polarity Earthly Branches: Rat, Tiger, Dragon, Horse, Monkey and Dog. In four pillars (BaZi) astrology a Wu day master is read as dependable and grounding, a stabilising presence for other elements in the chart, though the tradition also warns that too much Yang Earth without relief from Wood can read as stubbornness. Note that Wu is also, confusingly, the name of the seventh Earthly Branch (the Horse); the two are unrelated homophones written with different characters.