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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.^[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.

A vowel is a speech sound produced with a relatively open vocal tract, allowing its configuration to shape the sound without the complete closure or strong constriction characteristic of many consonants. Vowel height, vowel backness and lip rounding describe major articulatory differences. The vocal tract's acoustic resonances help determine the resulting formants.^[ref]

For example, [i] is a close front unrounded vowel, whereas [u] is close, back and rounded. Moving between them changes the tongue and lips, even if the fundamental frequency of vocal fold vibration is kept approximately constant. This separates vowel quality from pitch.

Many vowels occupy a syllable nucleus, but articulatory and syllabic descriptions answer different questions: a syllabic consonant can occupy that position too. Written vowel letters are also distinct from vowel sounds; the same spelling can correspond to different pronunciations.

Vowel backness describes how far forward or backward the principal tongue constriction lies during a vowel. Front, central and back are broad regions of this articulatory continuum. It is a separate descriptive dimension from vowel height and lip rounding.^[ref]

The close vowels [i] and [u] give a familiar front/back contrast, but they also differ in ordinary lip rounding. Comparing them alone consequently does not isolate the effect of vowel backness. Holding another articulatory dimension approximately fixed, or measuring it separately, makes a stronger experiment. A vowel chart summarizes configurations; it is not a literal scale drawing of every speaker's vocal tract.