A vowel is a syllabic speech sound pronounced without any stricture in the vocal tract. Vowels are one of the two principal classes of speech sounds, the other being the consonant. Vowels vary in quality, in loudness and also in quantity (length). They are usually voiced and are closely involved in prosodic variation such as tone, intonation and stress. The word vowel comes from the Latin word , meaning "vocal" (i.e. relating to the voice). In English, the word vowel is commonly used to refer both to vowel sounds and to the written symbols that represent them (a, e, i, o, u, and sometimes w and y). Definition There are two complementary definitions of vowel, one phonetic and the other phonological. In the phonetic definition, a vowel is a sound, such as the English "ah" or "oh" , produced with an open vocal tract; it is median (the air escapes along the middle of the tongue), oral (at least some of the airflow must escape through the mouth), frictionless and continuant. There is no significant build-up of air pressure at any point above the glottis. This contrasts with consonants, such as the English "sh" , which have a constriction or closure at some point along the vocal tract. In the phonological definition, a vowel is defined as syllabic, the sound that forms the peak of a syllable. A phonetically equivalent but non-syllabic sound is a semivowel. In oral languages, phonetic vowels normally form the peak (nucleus) of many or all syllables, whereas consonants form the onset and (in languages that have them) coda. Some languages allow other sounds to form the nucleus of a syllable, such as the syllabic (i.e., vocalic) l in the English word table (when not considered to have a weak vowel sound: ) or the syllabic r in the Serbo-Croatian word vrt "garden". The phonetic definition of "vowel" (i.e. a sound produced with no constriction in the vocal tract) does not always match the phonological definition (i.e. a sound that forms the peak of a syllable). The approximants and illustrate this: both are without much of a constriction in the vocal tract (so phonetically they seem to be vowel-like), but they occur at the onset of syllables (e.g. in "yet" and "wet") which suggests that phonologically they are consonants. A similar debate arises over whether a word like bird in a rhotic dialect has an r-colored