Casali (1995) defends the hypothesis that the proto-Volta–Congo language originally had a nine- or ten-vowel system employing vowel harmony and that this set has been reduced to a seven vowel-system in many Volta–Congo languages.
Casali (1995) defende a hipótese de que o proto-Volta-Congo tivera nove ou dez vogais e un sistema de harmonía vocálica e que posteriormente este inventario reduciríase a un sistema de sete vogais en moitas linguas Volta-Congo.