Ground-breaking studies on how Bangkok Thai tones have changed over the past 100 years (Pittayaporn 2007, 2018; Zhu et al. 2015) reveal a pattern that Zhu et al. (2015) term the “clockwise tone shift cycle:” low > falling > high level or rising-falling > rising > falling-rising or low. The present study addresses three follow-up questions: (1) Are tone changes like those seen in Bangkok Thai also attested in other languages? (2) What other tone changes are repeated across multiple languages? (3) What phonetic biases are most likely to be the origins of the reported changes? A typological review of 52 tone change studies across 45 Sinitic, Tai-Kadai, Hmong-Mien, and Tibeto-Burman languages reveals that clockwise changes are by far the most common. The paper concludes by exploring how tonal truncation (Xu 2017) generates synchronic variation that matches the diachronic patterns; this suggests that truncation is a key mechanism in tone change
