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