A monophthong (Greek monóphthongos from mónos "single" and phthóngos "sound") 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.The monophthongs can be contrasted with diphthongs, where the vowel quality changes within the same syllable, and hiatus, where two vowels are next to each other in different syllables.
The conversion of monophthongs to diphthongs (diphthongization) or of diphthongs to monophthongs (monophthongization), 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 English are, in fact, diphthongs; the vowel sound in pay - pronounced - is an example of this.Some dialects of English make monophthongs out of former diphthongs. For instance, Southern American English 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. For instance, the in words like pin changes 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.