17th Century Changes
By 1700 the following further developments had taken place:
- The monophthong /ɔː/ of soul was raised to /oː/, merging with boat (see tow–toe merger). This change did not happen before /r/.
- Short /a/ merged with /ɔ/ when following a /w/, as in want, quality. The merger was suppressed before a velar consonant, as in quack, twang, wag, wax. Before nonprevocalic /r/, the vowel was opened and lengthened, merging instead with /ɒː/, as in war.
- Short /ɔ/ had begun to partake in lengthening before a nonprevocalic voiceless fricative. This resulted in words like broth, cost, and off having /ɒː/ instead of /ɔ/.
- Short /ɒ/ before /r/ lengthened to /ɒː/: thus corn, /kɒːrn/
- In words such as change and chamber, the pronunciation /ɒː/ was gradually replaced in the standard language by a variant with /eː/, derived from Middle English /aː/. This explains the contemporary pronunciation of these words with /eɪ/.
This left the language with three low back vowels:
- /ɔ/ in dog and want.
- /ɔː/ in more.
- /ɒː/ in cause, and cost, and corn.
Read more about this topic: Phonological History Of English Low Back Vowels