The division in two parts, the most basic form of distinction, the 阴(陰)yin and 阳(陽)yang, is simple, but when you start to divide more, difficulties arise, There’s a current discussion if 少阳(少陽)shao yang means (bottom to top) undivided line and divided line or divided line and undivided lines and what represent shao yin; here you will see how complex is the simply division in 4 parts, Imagine the division in 64 parts of the Yi jing, it is 16 times more complex, and the division in 386 line readings makes it even more complex, but don’t be afraid, after you understand how to divide the process of dividing in more parts is simple( unfortunately we can’t say the same about the meaning of this divisions, they got extremely complex).
First I see two ways of doing this division, in much yang, yang, yin, much yin (the way it is not used today) or much yang, yang to yin, much yin and yin to yang(the way it’s used today). For example, in temperature you could divide in very hot as much yang, hot as yang, cold as yin and very cold as much yin(not used today), or, the method used today, very hot as much yang, mild but getting colder, yang to yin, very cold yin, and mild but getting warmer, yin to yang. For more concrete examples of cyclic movements, we could use the seasons: when your hemisphere is close to the sun it is summer, too much yang, when the axis of the earth become well distributed it is autumn, yang to yin, then your hemisphere will be farther from the sun and will be winter, and after that will be even again, and then it’s considered spring, yin to yang; pay attention that the only difference of autumn and spring is the direction their moving (what will come next), both are at the same level of the sun, receiving the same quantity of sun light, and light is yang, energy, heat. For example day and night, noon is much yang, afternoon yang to yin, night yin, mourning yin to yang; again, the mourning and the respective time of afternoon receive the same quantity of light, and produce the same angle of shadows, but one is going to yang and the other going to yin. This become even more difficult when we use non cyclic or semi cyclic movements.
Now the question, shao yang, yang to yin, is bottom line solid and upper line broken or the contrary (by discovering what is shao yang, 少阴(少陰)shao yin becomes evident). I, and, that I remember, Richard Wilhelm, favor the shao yang is bottom line solid and upper line is broken theory, against virtually everyone else. I will not simply say that I favor one or another but will explain why I favor this theory. First is the upward movement, for example when is totally yin, bottom line yin and upper line yin, the start of movement will go to yang, as when something becomes too much turns to the opposite direction, and a yang line will appear; it will appear on the bottom, because down is yang, yang is the beginning, past, the start, where things rise, are created; the once first yin line of tai yin太陰(old yin, very yin) will follow the natural process of going to the future, to the final, of continuing to rise, to the conclusion, and will move to the second place of shao yang, and the once second line will disapear, forming the shao yang: bottom line undivided and second line divided. The process will continue forming two yang lines in totally yang 太阳(太陽 tai yang, old yang, very much yang) and after that producing shao yin, yang to yin, bottom line divided, upper line undivided.
But since the upward movement may seem something new, although I explained it is not, I have a backup theory. It is said that the bagua, the eight trigrams, is formed by adding a third line on top of the four images, although they do not explain how is formed the four images( if we follow this theory of putting a line on top of a existing one, you would obtain exactly the 4 images as I explain they should be), when forming the trigrams you will see that the trigrams composed by a broken line on the bottom and a unbroken line on the top, that is, yin to yang, or shao yang, after added a third line on top creating 震zhen 雷lei thunder, and fire火huo 離li, using the 先天xian tian, pre heaven arrangement, they are in the left, following clockwise movement, that represents yin to yang, mourning, where the sun rise, east, spring, left (in north hemisphere, where is china), and the trigrams composed by the shao yin, wind and water, on the right, west, afternoon, fall, yang to yin. It’s not that lake or marsh represents spring to summer, heaven means summer, wind or wood is summer to fall (according to a diagram in the Richard Wilhelm book); lake and heaven represents summer, wind and water, the trigrams composed by the shao yin are fall, yang to yin, mountain and earth represents winter, and thunder and fire, the trigrams composed by shao yang, represents spring, yin to yang. Fits perfectly!