= Lip rounding
<Lip rounding> is the narrowing and reshaping of the lip opening, often with protrusion, used in producing some <vowels>. Rounded and unrounded configurations alter the acoustic filtering of the <vocal tract>. <Lip rounding> is distinct from <vowel height> and <vowel backness>, even though particular languages commonly associate these properties.<Essentials of Linguistics (2nd edition) by Catherine Anderson and others>[§3.5, Rounding and vowel descriptions]{ref}
Compare `[i]` with `[y]`: both are close front <vowels>, while `[y]` is rounded. A mirror makes the lip difference visible; a recording permits their <formants> to be compared. Keeping the intended <tongue> position and <pitch> similar helps separate the manipulation from simultaneous changes. The unrounded/rounded label is a concise category, while actual movements and the opening's geometry vary continuously.
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