A vowel is a sound in spoken langage (or a letter of the alphabet denoting such a sound) that has a sounding voice (vocal sound) of its own. Unlike a non-vowel (consonant), a vowel can be sounded on its own. A single vowel sound forms the basis of a syllable, although two adjacent vowel sounds can be blended together into a single syllable diphthong.
For those languages which some form of the Roman alphabet, such as English language, certain letters are identified as vowels because they are normally associated with vowel sounds. In the English language, the vowel letters are A , E , I, O , U and sometimes Y. In English, the letter W by itself is not usually a vowel, but can form a diphthong with the vowels A or O, and can serve as a vowel in a few Welsh-derived words like cwm and crwth.
There is necessarily not a direct one-to-one correspondance between the vowel sounds of a language and the vowel letters. Many languages that use a form of the Roman alphabet have more vowel sounds than can be represented by the standard vowel letters. In the case of English, the five primary vowel letters can represent both long and short vowel sounds (some of the long vowel sounds in English are actually diphthongs). Furthermore, English some vowel sounds are represented by combinations of vowel letters, sometimes in conjuction with, such as the ea in beat or the 'ough' in such words as through or thought.
Other languages also attempt to overcome the limitation in the number of Roman vowel letters in similar ways. Many languages, like English, make extensive use of combinations of vowel letters to represent various sounds. However, it is also very common for languages to add diacritical marks to vowels, such as accents or umlauts, to represent the variety of possible vowel sounds. Some languages have also constructed additional vowels that are based on the standard Roman vowels, such as æ or ø that are found in some of the Scandinavian languages.
Daniel Jones developed a system to describe vowels.