Note: This article deals with sound changes involving English-language diphthongs. Each of the following sound changes involved at least one phoneme which historically was a diphthong. The sound changes discussed here may also have involved a phoneme which was historically or is now a monophthong. For sound changes involving English-language centering diphthongs see English-language vowel changes before historic r.
The vein-vain merger is the merger of the Middle English diphthongs and that occurs in all dialects of present English. The merger was complete by perhaps the fourteenth century.
As a result of the merger, vein and vain are now homophones, but in early Middle English they were pronounced differently as and . Similarly day (from Old Englishdæġ) and way (from Old English weġ) did not rhyme before the merger.
The merged vowel was a diphthong, often transcribed . It later merged (in most dialects) with the of words like pane in the pane-pain merger.
Diphthongs of Late Middle English
The English of South-Eastern England in about 1400 had seven diphthongs: