The first word is called the upper word, and the second word is called the lower word. Take the initials of the first word and the finals of the second word.
For example, the word "Yin" takes the initial of "Yu" and the final of "Yan", which together is the ancient sound of the word "Yin".
The initial "Bi Qu Bi" and the final "Li" together are the ancient sounds of Bi.
Look, take the X, and bring Ian here, so the pronunciation here is Xian, which means that what we see here is actually a common interchangeable word.
This method, as well as a certain sound, is a common phonetic notation method in ancient Chinese and does not explain the meaning of the word.
It's not that the so-called reverse upstairs means return.