“Actually the degree of movement the tongue makes, when changing from an [ɑ] to an [ɔ], elicits an infinite variety of vowel changes between them, [ɑ] and [ɔ] are only parts of an outline in the over-all vowel spectrum. This may be said of the distance between any two vowels. There are literally thousands of possibilities, but our minds are trained to hear the relatively few sounds which make up that outline. For this reason, many voice teachers believe that no two singers can possibly sing the same vowels, as they are as variable as fingerprints.”

Leyerle, William D. Vocal development through organic imagery. Leyerle Pubns, 1986.