Yi

Yi is the second of the ten Heavenly Stems (tiangan). Its element is Wood and its polarity is yin, so Yi is glossed as yin Wood, likened in tradition to grass, vines, or flexible growing plants, yielding and adaptive where the yang Wood of Jia is the standing tree. As one of the ten stems it combines with the twelve Earthly Branches to build the sexagenary cycle of sixty used to number years, months, days, and double-hours in the traditional calendar. The pairing of the five elements with the two polarities gives the ten stems their nature: each of the five phases appears once as yang and once as yin. These are the conventions of the Chinese calendrical and cosmological tradition, drawn on in the almanac and in the art of fate calculation known as the four pillars.

Yi is the second Heavenly Stem, paired with Jia under the Wood phase but carrying the Yin polarity, traditionally likened to a climbing vine or bending grass rather than Jia's upright tree, supple and adaptable where Jia is rigid. As an even-numbered stem, Yi combines only with the six Yin-polarity Earthly Branches within the sixty-term sexagenary cycle, so a Yi year, month, day or hour is always paired with an Ox, Rabbit, Snake, Goat, Rooster or Pig branch rather than a Rat, Tiger, Dragon, Horse, Monkey or Dog one. In four pillars (BaZi) astrology a Yi day master is read as diplomatic and accommodating, able to grow around obstacles rather than through them, though the tradition also reads an excess of Yin Wood as indecision. The stem appears throughout classical and modern Chinese chronology, since sexagenary dating remained in administrative and calendrical use well into the twentieth century and is still printed on Chinese calendars today alongside the Gregorian date.

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. Yi pairs with Geng, 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.

Yi is the yin member of the Wood pair, and the conventional image contrasts it with Jia directly: where Jia is the timber tree, Yi is the vine, the grass and the flexible growth that finds its way around obstacles rather than through them. Both are Wood, and the pairing of a yang and a yin stem for each of the five phases is what brings the stems to ten.

The stem's own attributes here are Wood 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