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A vowel is a type of sound for which there is no closure of the throat or mouth at any point where vocalization occurs. This can be contrasted with consonants, which are sounds for which there are one or more points where air is stopped. In nearly all languages, words must contain at least one vowel. While, in English, a word can be formed without any consonants — such as the words I or a — no word may consist of only consonants.
In many languages, vowels are not crucial to the general meaning of the word. Rather, one in these languages — many of which are Semitic languages — acts more to give a specific inflection than to differentiate the word from other distinct words. A parallel of this in English can be seen in the example of dive and dove or lay and lie, in which the core word is the same, but the changed vowel denotes tense. Languages that have this type of structure often do not even mark all of these sounds in written text. Both Arabic and Hebrew are good examples of this, where the marking of many vowels is unnecessary in writing.
Since a vowel refers to a specific type of sound, some letters may orthographically represent a consonant in some circumstances, and a vowel in others. In English, this can be seen with the letters y and w, which are most often used to make consonant sounds but don't require the closure of the throat that's common to consonants. In the case of y, for example, people can compare its use in the words yonder and day. In the word yonder, it acts distinctly as a consonant, with the center of the tongue blocking the flow of air on one side by touching the palette of the mouth in what is called a palatal approximant. In the word day, on the other hand, it is forming a sound akin to if the word were written in English as dei.
In the case of w, the words woo and how can be examined. In the word woo, the letter is acting as a consonant, with the back of the tongue blocking the flow of air on one side by touching the palette of the mouth — what is called a labiovelar approximant. In the word how, it serves as a vowel, which could be represented in English writing as hao.
In English, there are five letters which always represent a vowel when written: a, e, i, o, and u. These five letters represent more than five sounds, however, depending on the word, or if they are combined with other letters. Readers can compare the letter a in the words hat and hate as one of many examples.