“Mutually explanatory characters is to describe a concept without actualcharacter by the way of borrowing characters with the same pronunciation.” Said in Hsu Shen’ s(許慎) masterpiece “Shuo Wen JieZi.”(說文解字) Duan Yu-tsai(段玉裁)put more emphasis on the linkage of pronunciation of two characters. However, there are examples contradictory to Duan’s theory in his book. Here we can get conclusion through analysis of relative documents.