How did Guang Yun's thirty-five letters evolve into thirty-six letters of the Song Dynasty?

The research on the evolution of Chinese initials is based on the initial system in the Middle Ages, and the initial system in Guang Yun evolved gradually.

1, Guang Yun has a kind of lip sound, and 36 letters are divided into light lips and heavy lips.

2. "Mud" and "Niang" in Guang Yun are combined into one initial, and the 36 letters are two different initial.

3. Pronunciation in Guang Yun is divided into two groups, with 36 letters as a group.

4. There are two vowels in the voiced throat of Guang Yun, and one vowel in the thirty-six letters.