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How does lip rounding change vowel sounds?

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Diagram of a human head in profile showing the location of vocal organs and possible places of articulation for speech sounds, with labels for lips, teeth, tongue, palate, uvula, pharynx, and vocal cords, and numbered regions corresponding to types of articulation such as bilabial, alveolar, and velar.
phonetics Diagram depicting the location of human vocal organs and possible places of articulation used for speech.

In phonetics, lip roundingā€”also known as labializationā€”is a primary articulatory dimension used to classify vowel sounds.12

When a vowel is rounded, the lips are shaped into a circular form during production. In contrast, unrounded vowels are produced with the lips in a slack or spread position, such as the ee sound in "meet". While there is a general cross-linguistic tendency for back vowels to be rounded and front vowels to be unrounded, this is not a universal rule; for instance, French includes high front rounded vowels.1

The degree of rounding can also vary, with terms like "close-rounded" describing sounds produced with significant lip rounding, such as the vowel in "boot". By altering the shape of the mouth, which acts as a resonance chamber, lip rounding changes the acoustic quality of the vowel produced.32