Vietnamese (Vietic, Mon-Khmer, Austroasiatic) is monosyllabic and tonal. Most Mon-Khmer (MK) languages are multisyllabic and atonal. Evidence suggests that Vietnamese (VN) has had its tones less than one millennium, and that other languages (both MK and non-MK) are also acquiring tones, a process called "tonogenesis". Comparing VN's phonology to that of closely related languages, Haudricourt has identified several stages of tonogenesis. We know "that" Vietnamese tonogenesis included monosyllabization, loss of onset voicing distinctions, and loss of laryngeal codas, but "how" and "why", and in what order? And why do other languages follow similar paths? Usage-based phonology claims that language change is driven by articulatory reduction, which motivates similar paths of change across related and unrelated languages. If speakers use a word often, they conserve effort by saying it the "easy" way. And even if a word is reduced, its frequency helps listeners to perceive it. Frequent words reduce first, then infrequent words. I argue that tonogenesis can be given such an articulatory account, and I detail what such an account should include. Comparing VN's sound changes to those in other MK languages, and to reductive processes noted by vocal physiology and articulatory phonetics, I demonstrate that each change along the tonogenetic path is an articulatory reduction. There is much about past tonogenetic processes that we cannot test, but the most recent of those processes are still synchronically evident in comparisons of frequent to infrequent words. To show that a usage-based account of tonogenesis is supported by synchronic facts, I analyze the spontaneous conversations and directed readings of 16 VN speakers in Hanoi and near Saigon. Acoustic measurements reveal that (relative to infrequent words) VN's frequent words are reduced in three ways: they exhibit less negative spectral tilt, less duration, and less pitch excursion. The significance of each is explained. Empirical results support the usage-based, articulatory account that I propose. Future studies of sound change will gain useful diachronic insights by including considerations of usage frequency and articulatory reduction. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]