Middle English
All of the above diphthongs came about within the Middle English era. Old English had a number of diphthongs, but all of them had been reduced to monophthongs in the transition to Middle English. Middle English diphthongs came about by various processes and at various time periods. Diphthongs tended to change their quality over time. The changes above occurred mostly between early and late Middle English. Early Middle English had a distinction between open-mid and close-mid diphthongs, and all of the close-mid diphthongs had been eliminated by late Middle English.