Sara-Bongo-Bagirmi (SBB) languages represent a group of some 40 African languages that are scattered between Lake Chad in the North-West, and Lake Albert in the South-East, thus covering parts of Chad, Sudan, South-Sudan, the Central African Republic, and the Democratic Republic of Congo. While languages in the East display two-tone systems directly reflecting the historical *SBB configuration, a large subgroup of ‘western’ (*OCC) languages later innovated in developing a new tone in the high frequencies. However this change followed different ways according to the grammatical category it affected: in verbs it is systematically correlated with the modification of a certain tone verb class while, in nouns, it represents non-systematic and irregular replacements of the original tone patterns resulting from a likely contact with a new linguistic environment.