|Sound change and alternation|
Monophthongization is a sound change by which a diphthong becomes a monophthong, a type of vowel shift. In languages that have undergone monophthongization, digraphs that formerly represented diphthongs now represent monophthongs. The opposite of monophthongization is vowel breaking.
Some English sounds that may be perceived by native speakers as single vowels are in fact diphthongs; the vowel sound in pay — pronounced /ˈpeɪ/ — is an example of this. However, in some dialects (e.g. Scottish English) /eɪ/ is a monophthong [e].
In Received Pronunciation, when a diphthong is followed by schwa, the diphthong may change to a monophthong. For example, the citation form of the word our is /ˈaʊə/, but in speech it is often pronounced as [ɑə] or even as a monophthong [ɑː]. This process is sometimes called smoothing.
In Sanskrit, the sounds pronounced as /e/ and /o/ are written as ai and au in 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.
Greek underwent monophthongization at many points during its history. For instance, the diphthongs /ei ou/ monophthongized to /eː oː/ around the 5th century BC, and the diphthong /ai/ monophthongized to /eː/ in the Koine Greek period. For more information, see Ancient Greek phonology § Monophthongization and Koine Greek phonology.
Classical Arabic has two diphthongs, /ai/ and /au/. In the majority of modern Arabic dialects, these diphthongs are realised as the long vowels /e/ and /o/, respectively. One notable exception is the Lebanese dialect, which preserves the original pronunciations.