First, according to the tone, the last word in the upper part is flat, three tones and four tones are flat, the last word in the lower part is flat, and one tone and two tones are flat.
Second, according to causality, "cause" is the upper part and "effect" is the lower part, because there is cause first, then there is effect.
Third, in terms of time, time is the first part and time is the second part.
Fourth, according to the spatial scope, the small ones are generally in front and the big ones are behind.