The second year is listed as yi chou, the third as bing yin, the fourth as ding mao, and so on, through gui you. The eleventh year through the twentieth year are listed as jia xu, yi hai, bing zi, and so on, through gui wei. And so forth.
The listing of the years along the dual sequences of stems and branches is not simply an enumeration. Because the stems and branches themselves are categorized as yin and yang (with stems and branches carrying uneven numbers being yang and those carrying even numbers being yin; yang stems can be combined only with yang branches, and yin stems can be associated only with yin branches) and because the stem and branch combinations associated with each year constitute pairings of different yin-yang categories with the five agents, they refer to a very specific qualitative meaning. That is, once the stem and branch designation of a given year is known, its climatic characteristics can be calculated through the associations of the respective stem and branch with the five periods and six qi. (See tables 1 and 2.)
THE FIVE PERIODS
“Five periods, ” wu yun $, is a comprehensive designation referring to the nature or quality of periodically recurring phenomena, including qi, associated with the “five agents” (wu xing $), metal, wood, water, fire, and soil, as they may characterize consecutive years or the five seasons in the course of one year. That is, one “period” may refer to one entire year or to one of the five seasons.
The five periods in the doctrine of the five periods and six qi are closely related to the five agents in the doctrine of the five agents, and the seven comprehensive discourses of the Su wen go to great lengths to introduce concepts of the doctrine of the five agents in the context of the passage of the five periods. For example:
The east generates wind. Wind generates wood. Wood generates sour [flavor]. Sour [flavor] generates the liver. The liver generates the sinews. The sinews generate the heart….