A monophthong (Greek μονόφθογγος, "monophthongos" = single note) is a "pure" vowel sound, one whose articulation at both beginning and end is relatively fixed, and which does not glide up or down towards a new position of articulation; compare diphthong.
In the English language, there are in practice relatively few monophthongs. The position, beginnings, and endings of vowel articulation are perhaps the chief distinguishing feature among the various dialects of English; the differences between the pronunciations of British English and American English are largely a result of the different realization of vowel sounds. The conversion of monophthongs to diphthongs, or of diphthongs to monophthongs, is a major element of language change and is likely the cause of further changes. Some sounds that may be perceived by native speakers as monophthongs in both these varieties of English are, in fact, diphthongs; the vowel sound in pay - is an example of this. Some dialects of English make monophthongs out of former diphthongs. The vowel boat is generally realized as a diphthong or . Also, the speech of the southern United States tends to alter the diphthong as in eye to an somewhere between and . On the other hand, former monophthongs have become diphthongs in American English such as the in words like pin changing to in some American dialects.
Historically, some languages treat vowel sounds that were formerly diphthongs as monophthongs. Such is the case in Sanskrit, in whose grammar the sounds now realised as and are conceptually ai and au, and are written that way in the Devanagari and related alphabets. The sounds and exist in Sanskrit, but are written as if they were āi and āu, with long initial vowels. Similar processes of the creation of new monophthongs from old diphthongs are preserved in the traditional spellings of languages as diverse as French and modern Greek.