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|Sound change and alternation
The best-known example in the English language is the Great Vowel Shift, which began in the 15th century. The Greek language also underwent a vowel shift near the beginning of the Common Era, which included iotacism. Among the Semitic languages, the Canaanite languages underwent a shift in which Proto-Semitic *ā became ō in Proto-Canaanite (a language likely very similar to Biblical Hebrew).
A vowel shift can involve a merger of two previously different sounds, or it can be a chain shift.