String pair means that in a verse or couplet, the sentence and the pair are not opposite in meaning and grammatical structure, but are inherited from top to bottom. The two sentences cannot be separated from each other or reversed. There is a certain sequence in their language structures.
If you want to cross the mountain of knowledge, you must take the road of diligence. If you want to cross the ocean of knowledge, you must use diligence as a boat to cross the sea is a string pair according to the above definition.