A monophthong (in Greek μονόφθογγος = 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 deep gulf between British English and American English is a result of the different realization of vowel sounds. The conversion of monophthongs to diphthongs, or of diphthongs to monophthongs, is a major cause of language change. Some sounds that are (arguably) perceived as monophthongs in both these varieties of English are in fact diphthongs, such as the vowel sound in pay — IPA or /peɪ/. In British English, the sound of /o/ as in boat tends to become a diphthong /əʊ/; American English has either /ow/ with much less change of articulation or a pure vowel /o/ in this position. On the other hand, some dialects of English make monophthongs out of former diphthongs, such as the speech of the southern United States, which tends to alter the diphthong /aj/ as in eye to a vowel sound somewhere between /ɑ/ and /æ/. Another new diphthong that has arisen from a former monophthong can be heard in some American English pronunciation of words like pin, which is pronounced /ɪə/.
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 /e/ and /o/ are conceptually ai and au, and are written that way in the Devanagari and related alphabets. The sounds /ai/ and /au/ 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.