The first Earthly Branch in the traditional Chinese cycle is Zi, written 子. It is the opening symbol in the sequence of 12 Earthly Branches and corresponds to the Rat in the Chinese zodiac.
The Earthly Branches are an ancient ordered system used with the 10 Heavenly Stems in the sexagenary cycle. They also mark years, months, days, directions, and traditional double-hours. Zi occupies the first position and is associated with the period from 11 p.m. to 1 a.m., the midnight interval in traditional timekeeping.
The next branches are Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, and Hai. Their animal associations are useful memory aids, but the branches themselves are Chinese ordinal symbols, not simply animal names.
One common mix-up is choosing Yin, because Yin is the third branch and is strongly associated with the Tiger. Zi is also important in the 60-part sexagenary cycle: Jia-Zi, the first Stem-Branch pairing, begins with this branch.