The language may be tonal or non-tonal. At first glance, it seems that the transition of a language from one type to another is hardly possible. However, the study of historical changes in the languages of the Mainland Southeast Asia (MSEA) shows that the transition from the non-tonal language to the tonal language is still possible. The global process, which determines the evolution of the MSEA languages, regardless of their genetic affiliation, is the monosyllabisation that means the restructuring of polysyllabic languages into monosyllabic ones through the sesquisyllabic stage. The development of phonation-type registers and tones as in the Vietic branch of the Austroasiatic languages is the result of the monosyllabisation process. Speakers of the most Vietic languages, except Vietnamese and Muong, are small groups of recent hunter-gatherers living in the mountains of Vietnam and Laos. Based on the materials of Vietic languages, we can trace the different stages of the monosyllabisation, and the development of various types of prosodic systems, including tonal and post-register ones.