Cot-caught merger

The cot–caught merger (also known as the low back merger or the LOT–THOUGHT merger) is a phonemic merger that has taken place in some varieties of English, between the phonemes which are conventionally represented in the IPA as /ɔː/ (as in caught and thought) and /ɒ/ (as in cot and lot). In varieties in which the merger has taken place, including a few in the British Isles and many in North America, what were historically two separate phonemes have fallen together into a single sound, so that caught and cot are pronounced identically.

The shift causes the vowel sound in words like cot, nod and stock and the vowel sound in words like caught, gnawed and stalk to merge into a single phoneme; therefore the pairs cot and caught, stock and stalk, nod and gnawed become perfect homophones, and shock and talk, for example, become perfect rhymes. The merger occurs in:

Nowhere is the shift more complex than in North American English. The presence of the merger and its absence are both found in many different regions of the North American continent, where it has been studied in greatest depth, and in both urban and rural environments. The symbols traditionally used to transcribe the vowels in the words cot and caught as spoken in American English are /ɑ/ and /ɔ/, respectively, although their precise phonetic values may vary, as does the phonetic value of the merged vowel in the regions where the merger occurs.

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