Full–fool merger

In the history of English phonology, there have been many diachronic sound changes affecting vowels, especially involving phonemic splits and mergers. A number of these changes are specific to vowels which occur before /l/.

Diphthongization occurred since Early Modern English in certain -al- and -ol- sequences before coronal or velar consonants, or at the end of a word or morpheme. In these sequences, /al/ became /awl/ and then /ɑul/, while /ɔl/ became /ɔwl/ and then /ɔul/. Both of these merged with existing diphthongs: /ɑu/ as in law and /ɔu/ as in throw.

At the end of a word or morpheme, this produced all, ball, call, fall, gall, hall, mall, small, squall, stall, pall, tall, thrall, wall, control, droll, extol, knoll, poll, roll, scroll, stroll, swollen, toll, and troll. The word shall did not follow this trend, and remains /ʃæl/ today.

Before coronal consonants, this produced Alderney, alter, bald, balderdash, false, falter, halt, malt, palsy, salt, , , bold, cold, fold, gold, hold, molten, mould/mold, old, shoulder (earlier sholder), smolder, told, and wold (in the sense of "tract of land"). As with shall, the word shalt did not follow this trend, and remains /ʃælt/ today.

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