The standard Lexical sets for English were introduced by professor John Wells which are widespread. Each lexical set represents a vowel present in a number of words, for example: the THOUGHT vowel /ɔː/ is found in taught, sauce, hawk, jaw, broad. The GOAT vowel /əʊ/ is found in soap, joke, home, know, so, roll etc.
However, there are three lexical sets NORTH, FORCE and THOUGHT for the same vowel /ɔː/ especially in British English. He has included different words in each lexical set:
- THOUGHT: taught, sauce, hawk, jaw, broad
- NORTH: for, war, short, scorch, born, warm
- FORCE: four, wore, sport, porch, borne, story
(All the above information comes from Wikipedia)
All those words sound the same to me (a non-native speaker of English who is learning British English). I can't detect any difference between them and looking the sets up on UCL website, there is no information on whether there is any difference between them or not.
Why did John Wells need three lexical sets for the same vowel when he could easily have incorporated those words in one lexical set (say for example THOUGHT)? Are those vowels--NORTH, FORCE and THOUGHT--different in British English? Can anyone provide the phonetic values/realizations of those vowels they have in British English?
I emailed professor John Wells and asked him why there were three Lexical sets for the same vowel. I said to him "someone (I meant @Nardog) told me that the whole point of lexical sets is to make it easier to describe differences between [vowels in different] accents. He replied to me and said:
"Someone" is correct. And not only American English, but also Scottish English and various other varieties. Read the book!