The mechanism of formation of “N+ㅅ+N” compounds in Middle Korean corresponds to the lexicalization of syntactic composition and the analysis of the sait-sori phenomenon, which was caused by the phonetic change of the genitive “ㅅ” in the 15th century. In Modern Korean, the genitive “ㅅ” disappeared, remaining in many words as a form of trace. Thus, the sait-sori phenomenon became a motive of analysis as the mechanism of formation of the “N+ㅅ+N” compound. The trace of the genitive “ㅅ” remained even after its disappearance, causing modern Korean speakers to carry a memory burden. In addition, the analysis of the sait-sori phenomenon expanded its domain of work and reproduced many “N+ㅅ+N” compounds. As a result, the appearance of the modern Korean sai-siot became unexpected and irregular.