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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>.<Essentials of Linguistics (2nd edition) by Catherine Anderson and others>[Section3.5, Describing vowels]{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 (source code)