This chapter discusses phonological motivations for morpho-syntactic changes in history. In general, Old Chinese was monosyllabic, which means that the overwhelming majority of words were represented by a single syllable, regardless of whether they were content or function words. In Middle Chinese, the phonological system was dramatically simplified; the number of consonants and vowels was reduced, and the syllabic structures were simplified. To restore the phonological distinctions of lexical items, the language increased the number of syllables for words, typically by adding one syllable to originally monosyllabic words. This disyllabification tendency has lasted nearly two millennia since then. This new sort of prosodic unit stimulated the fusion of two monosyllabic items, a key factor for the emergence of the resultative construction and other grammatical morphemes.