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.