Zi is the first of the twelve Earthly Branches (dizhi), the cyclical signs that combine with the ten Heavenly Stems to form the sexagenary cycle of sixty. Its zodiac animal is the Rat, its element is Water, its direction is due north, and its double-hour runs from 11 pm to 1 am, the hour of midnight, when one day turns to the next. As the first branch, Zi opens the cycle of the twelve, and the year, month, day, and hour it names carry the Rat's associations in the almanac. The branches order the twelve-year animal cycle, the twelve months, and the twelve double-hours of the day. These are the conventions of the Chinese calendrical and cosmological tradition; the twelve branches, paired with the ten stems, recombine only after sixty steps, giving the great cycle its length.
Zi is the first of the twelve Earthly Branches (Dizhi), attested alongside the Heavenly Stems on Shang-dynasty oracle bones, and it corresponds to the Rat in the twelve-animal cycle used to name years, and, in the traditional Chinese double-hour system, to the two hours around midnight, 11 pm to 1 am, when the day is reckoned to turn over. Through the theory of hidden stems, by which each branch is said to harbour one or more of the ten Heavenly Stems within it and so carry its own elemental association, Zi's primary hidden stem is Gui, giving it the Water phase; it is also assigned the north as a fixed direction. Zi pairs with the ten stems within the sixty-term sexagenary cycle, so a Zi year always falls with a Yang-polarity stem, Jia, Bing, Wu, Geng or Ren, never with a Yin one.