Jia is the first of the ten Heavenly Stems (tiangan), the cyclical signs that combine with the twelve Earthly Branches to form the sexagenary cycle of sixty, the backbone of the traditional Chinese calendar for numbering years, months, days, and hours. Its element is Wood and its polarity is yang, so Jia is often glossed as yang Wood, likened in tradition to a great tree or standing timber, upright and pioneering. In the sixty-year cycle each stem pairs in turn with a branch, and because ten and twelve share a common factor the stems and branches recombine only after sixty steps. The stems also carry associations of direction and the five phases used in almanac reckoning and fate calculation. These are the conventions of the Chinese calendrical and cosmological tradition, in which Jia opens the count of stems.
Jia is the first of the ten Heavenly Stems (Tiangan), a cycle first attested on Shang-dynasty oracle bones of the late second millennium BCE, where it was already in use for naming days. Paired with Yi, Jia carries the Wood phase, and as the odd-numbered, first member of that pair it takes the Yang polarity, conventionally likened to standing timber or a tall tree, active and upward-growing, against Yi's Yin Wood, likened to a pliant vine. The ten stems combine with the twelve Earthly Branches (Dizhi) to form the sexagenary cycle of sixty paired terms, the basic reckoning of years, months, days and hours in the traditional Chinese calendar and its continuing use in four pillars (BaZi) astrology; because ten and twelve share a lowest common multiple of sixty rather than pairing stem to branch one-to-one, a stem always recurs with the same Yin or Yang branches, so Jia never falls on an even-numbered branch.