The sound change turns the vowel in cot (SAMPA /A:/ in American English) and caught ( /O:/ ) into a single sound, usually close to /A:/, so that the two words become homophones. This sound change appears to have occurred at some time in the nineteenth century; it is regular in most of inland English-speaking North America, so that only a few areas, most of which are along the east coast of the continent, continue to observe the distinction. Some varieties of North American English still have both the vowels [A:] and [O:], but [O:] is a conditioned variant that only occurs before certain sounds, particularly /r/ or /l/, and does not count as a separate phoneme.
For most speakers of North American English, the two sounds [A:] and [O:] are allophones; they do not perceive differences in their usage, and hear the distinct vowels used by speakers whose dialects do distinguish them as variations on the same vowel. They hear the broad A of British English as the same, single vowel sound. But in British English, in particular in Received Pronunciation, there are three sounds distinguished: the long /A:/ of cart, the long /O:/ of caught, and the short rounded /Q/ of cot. The American cot-caught vowels have merged onto what the Briton hears as the vowel of cart.