The traditional model of contour tone distribution uses phonological units to describe which syllable types can bear rising or falling tones. A common pattern is one in which contour tones are permitted on long vowels but not on short vowels; this is explained in the traditional model by assuming that the mora is the tone-bearing unit and there is a one-tone-per-mora restriction that prohibits contour tones on short vowels. An alternative to the phonological approach is one in which contour tone distribution is phonetically based. In the phonetic model, there is no role for the mora in determining contour tone distribution; instead, it is duration and/or sonority that determine whether a contour can occur on a given syllable. This paper shows how Luganda bears on the choice between the two models, favoring the phonological approach over the phonetic one. The mora turns out to be crucial to explaining contour tone distribution in Luganda, while the duration-based phonetic model makes incorrect predictions for this language.