Geng is the seventh of the ten Heavenly Stems (tiangan). Its element is Metal and its polarity is yang, so Geng is glossed as yang Metal, likened in tradition to a sword, an axe, or wrought iron, hard, sharp, and forceful. 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. The stems run in fixed order, each bearing one of the five phases in a yang or yin form, and Geng brings the yang aspect of Metal after the two Earth stems. These are the conventions of the Chinese calendrical and cosmological tradition, used in the almanac and in the four-pillars method of reckoning a person's fate.
Geng is the seventh Heavenly Stem, the first of the pair carrying the Metal phase, and as an odd-numbered stem it takes the Yang polarity, traditionally likened to raw ore, a sword blade or an axe, hard and unrefined, in contrast to Xin's Yin Metal, likened to a fine blade or jewellery. In the controlling cycle of the five phases, Metal is checked by Fire and in turn checks Wood, a relationship the tradition reads onto a Geng day master in four pillars (BaZi) astrology as decisive and forceful, sometimes to the point of bluntness, a personality suited to cutting through obstacles rather than negotiating around them. Within the sixty-term sexagenary cycle, Geng combines only with Yang-polarity Earthly Branches: Rat, Tiger, Dragon, Horse, Monkey and Dog. Geng years recur every ten years and, combined with the branch cycle, only every sixty years with the same paired branch, the basis of the traditional Chinese reckoning of a sixty-year lifespan as a complete cycle.