The tonal system of Bora, a Witotoan language of Colombia and Peru, has high and low tones. Most tonal phenomena refer to low—not high—tone. For example, virtually all lexically marked tones are low. Further, many suffixes assign low tones to preceding syllables, but never creating a sequence of adjacent low tones. This is modeled with the cyclical addition of suffixes where adjacent low tones are avoided by either blocking the placement of the suffix's low tone or by delinking a previously assigned low tone. Finally, low tone plays a role in various syntactic constructions.
Weber, David J. and Thiesen, Wesley
"A synopsis of Bora tone,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 45, Article 2.
Available at: https://commons.und.edu/sil-work-papers/vol45/iss1/2