Hai is the twelfth and last of the Earthly Branches (dizhi). Its zodiac animal is the Pig, the Boar in the Japanese version, its element is Water, its direction is north-northwest, and its double-hour runs from 9 to 11 pm, the late evening before midnight. As the final branch it combines with the ten Heavenly Stems to complete the sexagenary cycle of sixty; after Hai the count of branches returns to Zi and the Rat. The branch names the last animal of the twelve-year cycle and the corresponding month and double-hour, carrying the Pig's associations of abundance and ease in the almanac. These are the conventions of the Chinese calendrical and cosmological tradition, in which twelve branches and ten stems together structure the reckoning of years, months, days, and hours.
Hai is the twelfth and final Earthly Branch, corresponding to the Pig, and to the double hour from 9 pm to 11 pm, closing the day before it turns over into Zi at midnight. Its dominant hidden stem is Ren, giving it the Water phase, with Jia present as a secondary hidden stem, so that Hai and Zi together anchor the Water portion of the twelve-branch wheel at the point where the cycle both ends and begins again. Hai is a Yin-polarity branch, pairing only with Yi, Ding, Ji, Xin or Gui within the sexagenary cycle, and it governs the tenth lunar month. Because twelve branches and ten stems share a lowest common multiple of sixty rather than twelve, a Hai year does not recur with the same paired stem for another sixty years, the same logic that gives the whole sexagenary system its traditional association with a complete human lifespan. In BaZi astrology a chart with Hai prominent is read through the Pig's traditional associations of generosity, honesty and good fortune.