Si is the sixth of the twelve Earthly Branches (dizhi). Its zodiac animal is the Snake, its element is Fire, its direction is south-southeast, and its double-hour runs from 9 to 11 am, the late morning. As one of the twelve branches it combines with the ten Heavenly Stems to build the sexagenary cycle of sixty used to number years, months, days, and double-hours in the traditional calendar. The branch names the sixth animal of the twelve-year cycle and the corresponding month and double-hour, carrying the Snake's associations of wisdom and subtlety in the almanac. These are the conventions of the Chinese calendrical and cosmological tradition, in which the twelve branches, paired with the ten stems, recombine only after sixty steps to give the great cycle its length.
Si is the sixth Earthly Branch, corresponding to the Snake, and to the double hour from 9 am to 11 am, the later morning. Its dominant hidden stem is Bing, giving it the Fire phase, with Wu and Geng present as secondary hidden stems. Si is a Yin-polarity branch, pairing only with Yi, Ding, Ji, Xin or Gui within the sexagenary cycle, and it governs the fourth lunar month. Note that Si the branch and Si the given cardinal direction south (written with a different character in some regional systems) are sometimes confused by learners; the branch Si is not itself a compass direction, though its Fire phase associates it with the general southern quadrant of the branch-direction wheel. In BaZi astrology a chart with Si prominent is read through the Snake's traditional associations of intelligence, subtlety and a private, watchful temperament, paired with the Fire phase's quickness and capacity to illuminate what the Snake's caution would otherwise keep hidden.