All vowel distinctions reduce to tongue position, openness, and lip rounding

Evidence

Vowels can be classified using a few articulatory features: • Front–back position of articulation as a measure of tongue position. For example, in the words bet and but, the only change in articulation is that the position of the tongue moves from the front in bet to the back in but. • Open–close dimension (openness) of articulation specifies the up–down position of the tongue. The closer the tongue is to the palate, the more the vowel is ‘closed’. • Rounded–unrounded shape of the opening of the lips. If a vowel is pronounced first with lips [laterally] wide and then with rounded lips, the perceived vowel changes, as for the vowels in the words but and caught. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 574

Categorization

• XREF: These three articulatory dimensions map directly onto the vowel space used in formant-frequency plots and speech synthesis parameterization.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 574