The sounds heard in modern English were significantly influenced by the Great Vowel Shift, as well as more recent developments in some dialects such as the cot–caught merger.
The corresponding spellings were ⟨a⟩ and ⟨o⟩, with the length distinctions not normally marked; in modern editions of Old English texts, the long vowels are often written ⟨ā⟩, ⟨ō⟩.
[5][6] Examples of possible homophones resulting from the merger include Khan and con (/kɑn/) as well as Saab and sob (/sɑb/).
[5][6] The LOT–CLOTH split is the result of a late 17th-century sound change that lengthened /ɒ/ to [ɒː] before voiceless fricatives, and also before /n/ in the words gone and sometimes on.
Some words that entered the language later, especially when used more in writing than speech, are exempt from the lengthening, e.g. joss and Goth with the short vowel.
As a result of the lengthening and raising, in the above-mentioned accents cross rhymes with sauce, and soft and cloth also have the vowel /ɔː/.
The lengthening and raising generally happened before the fricatives /f/, /θ/, and /s/, and in the word water for an unknown reason (compare the broadening of a in father).
In American English, the raising was extended to the environment before velars /ŋ/ and /ɡ/, and sometimes before /k/ as well, giving pronunciations like /lɔŋ/ for long, /dɔɡ/ for dog and /ˈtʃɔklət/ for chocolate.
Its actual phonetic realization may be open [ɒ], whereas the lot vowel may be realized as central [ä].
There are also significant complexities in the pronunciation of written o occurring before one of the triggering phonemes /f θ s ŋ ɡ/ in a non-final syllable.
The isogloss for this difference, termed the ON line, lies between New York City and Philadelphia on the East Coast and runs West as far as speakers without the merger can be found.
[22] In broad Geordie, some THOUGHT words (roughly, those spelled with a, as in walk and talk) have [aː] (which phonetically is the long counterpart of TRAP /a/) instead of the standard [ɔː].
[aː] is therefore not necessarily a distinct phoneme in the vowel system of Geordie, also because it occurs as an allophone of /a/ before voiced consonants.
It corresponds to /æ/, /ɒ/, /ɔː/ and (when not prevocalic within the same word) /ɑːr/ and even /ɔːr/ in other dialects: For the sake of simplicity, instances of an unrounded LOT vowel (phonetically [ɑ]) that do not merge with PALM/START are excluded from the table below.