Ancient Chinese people had the skill to calculate the history of five thousands years and record the chronological sequence of events. They invented a measurement by two sets of signs in a certain sequence - Heavenly Stems and Earthly Branches.

Heavenly Stems indicate 10 circulatory signs - jia, yi, bing, ding, wu, ji, geng, xin, ren, and gui; and the Earthly Branches refers to zi, chou, yin, mao, chen, si, wu, wei, shen, you, xu, and hai.

As the Heavenly Stems take the decimal system to account, and the Earthly Branches a duodenary system with the advantage of noting the 12 months; the combination of these two sets became sexagesimal one. Thus came a way of counting time which has been passed down through thousands of years. A year can be represented by the composition of two characters, one from the heavenly set, and one from the other. Take the year of 2005 for example, 2005 can be described as 'Yi You', a combination of the second sign in Heavenly Stems and the tenth one in the Earthly Branches. Then the next year 2006 will be 'Bing Xu' - the third sign and the eleventh one's association. Some one has given a vivid comparison that the 10 signs of Heavenly Stems and the 12 components of the other set are just like two gear wheels of different size.

Eight Characters or 'ba zi' in Chinese of ancient China was very popular and mostly used in fortune-telling. In four pairs of year, month, day, and hour of a person's birth, each pair including one Heavenly Stem and one Earthly Branch, this expression has always been vital especially when used for foretelling a new couple's fate before their marriage.

Earthly Branches can be used to count time of years, months and hours. When counting years, people use them with the 12 Chinese zodiacs and strictly one Earthly Branch corresponds to one fixed zodiac. For example, our ancestors have divided a day and night into twelve sections and named each as a 'shi chen', which is equal to today's two hours and has a fixed Earthly Branch name. From 11 pm at night to 1 am is decided to be the first one - 'zi shi', then the second two hours 'chou shi', on the analogy of this.