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It is a tonal language with four tones: high, low, falling and rising. Downstep can occur between two high tones or between a high and falling tone. A contour (rising or falling) tone can occur on a vowel if and only if the vowel is the realization of two underlying vowels.
Laks, Bernard; Durand, Jacques; Goldsmith, John (2002). Phonetics, phonology, and cognition. Oxford: Oxford University Press. pp. 80–83. ISBN9780198299837.