Archeological discoveries show that ancient people in China began to use theCelestial Stemand theTerrestrial Branchto record time as early as the Yin and Shang period about 4,000 years ago.

Within each 60-year cycle, each year is assigned name consisting of two components:

The first component is aCelestial Stem. These words areJia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, GuiinPingyinand they have no English equivalents.

The second component is aTerrestrial Branch. The names of the corresponding animals in the zodiac cycle of 12 animals are given in parentheses. They areZi(rat),Chou(ox),Yin(tiger),Mao(hare, rabbit),Chen(dragon),Si(snake),Wei(sheep),Shen(monkey),You(rooster),Xu(dog) andHai(pig).

Each of the two components is used sequentially. Thus, the 1st year of the 60-year cycle becomesJia-Zi, the 2nd year isYi-Chou,the 3rd year isBing-Yin, etc. When we reach the end of a component, we start from the beginning: The 10th year isGui-You, the 11th year isJia-Xu(restarting theCelestial Stem), the 12th year isYi-Hai, and the 13th year is Bing-Zi(restarting theTerrestrial Branch). Finally, the 60th year becomesGui-Hai.

This way of naming years within a 60-year cycle goes back approximately 2000 years. A similar naming of days and months has fallen into disuse, but the date name is still listed in calendars.

It is customary to number the 60-year cycles since 2637 B.C.E., when the calendar was supposedly invented. In that year the first 60-year cycle started.