The sixty-year stems and branches are cyclical, and the Yinmao is wood, which originates in spring. Through the cycle of the twelve branches of the four seasons, the first month is exactly Yin. Therefore, the ancients concluded that the first month is Jian Yin and the second month is Mao. Why not use the sub-character at the beginning? According to the seasonal changes of the twelve branches, no matter how you arrange it, you can't arrange the sub-characters to the first month. How about you give it a try. If you read the perpetual calendar, you will find that it is always a cycle. This is the changing law of yin and yang and the five elements in the four seasons.
I hope the above answers can help you.