Xin

Xin is the eighth of the ten Heavenly Stems (tiangan). Its element is Metal and its polarity is yin, so Xin is glossed as yin Metal, likened in tradition to jewellery, fine ornament, or a small refined blade, delicate and precise where the yang Metal of Geng is the axe. As one of the ten stems it pairs with the twelve Earthly Branches to build the sexagenary cycle of sixty used to number years, months, days, and double-hours. Each of the five phases appears among the stems once as yang and once as yin, and Xin carries the yin aspect of Metal. These are the conventions of the Chinese calendrical and cosmological tradition, drawn on in the almanac and in the four-pillars art of fate calculation from the stems and branches of a birth.

Xin is the eighth Heavenly Stem, paired with Geng under the Metal phase but carrying the Yin polarity, likened to a finished blade, jewellery or fine ornament rather than Geng's raw ore, refined and precise where Geng is blunt force. As an even-numbered stem, Xin combines only with the six Yin Earthly Branches, Ox, Rabbit, Snake, Goat, Rooster and Pig, within the sixty-term sexagenary cycle that names years, months, days and hours. In four pillars (BaZi) astrology a Xin day master is read as meticulous and self-possessed, valuing precision and appearance, and the tradition associates the stem with justice and with cutting instruments used for careful work rather than for force. The Metal phase as a whole is correlated in the Huangdi Neijing with the lungs, the pungent taste, the colour white, the direction west and the season of autumn, correlations that carry over into how Xin is read in traditional Chinese medicine's constitutional typologies.

Sources Chinese Mathematical Astrology: Reaching Out to the StarsHo Peng Yoke with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown

Every stem has a partner among the other nine, and the five pairings, called the combinations of the heavenly stems, are fixed: each stem combines with the one five places away from it. Xin pairs with Bing, and the tradition holds that a combined pair takes on a phase of its own distinct from either stem's native phase. The pairing is used in almanac work and in fate calculation, where a combination in a chart is read as a binding between the two positions that carry it.

The ten stems are older than the sexagenary cycle they now form half of. On the Shang oracle bones of the second millennium BCE they name the ten days of the xun, the Shang week, and Shang kings are known to history by posthumous names built on them, so a stem was a unit of the working calendar before it was a term of cosmological art. The five-phase and yin-yang readings attached to the stems are later, systematised under the Han, and the atlas records the two layers as two layers.

Xin is yin Metal, the finished and worked object against the raw or edged metal of Geng, and the images the tradition uses are jewellery and fine instruments. The eighth stem completes the Metal pair, and the two Water stems that follow close the run of ten before it begins again at Jia.

The stem's own attributes here are Metal and yin polarity. Those two facts fix its place in the run of ten, since the stems proceed through the five phases in generation order taking the yang member first and the yin member second, and they fix how it is read wherever it appears, in the year, the month, the day or the double hour of a Four Pillars chart.

Sources Chinese Mathematical Astrology: Reaching Out to the StarsHo Peng Yoke with The Chinese Sexagenary Cycle and the Ritual Origins of the CalendarAdam Smith