A current debate going on among Yoruba linguists is the existence and phonological status of the high non-expanded vowels. Indeed while Igala, Isekiri and many Yoruba dialects exhibit a seven-vowel system, other Yoruba dialects exhibit a ·nine-vowel system (including i and o), both types showing however interesting vowel co-occurrence restrictions. Given this situation, some scholars argue that proto-Yoruba and proto-Yorubold had a nine-vowel system with cross-height vowel harmony, while others claim that they had a seven-vowel system. The present paper reviews both positions and suggests another alternative; in particular it shows how cross-height vowel harmony came about in Yoruboid.