Formerly, all dialects of the English language had the sound of /æ/ as in cat ( /kæt/ ) in all of these words. At some time, probably during the late eighteenth century, a sound change occurred in southern England that changed the sound of /æ/ to SAMPA /A:/ in words in which the former sound appeared before /f, s, th, v, z/, either alone or in the company of /m/ or /n/. British pahth for path and vahz for vase are the result of this sound shift. It does not occur before other consonants; thus British English, like American English, preserves the contrasting "flat a" in words like cat. The broad A was considered sub-standard or "Cockney" as recently as 1900 but is now a characteristic of the prestige Received Pronunciation and other accents now considered standard.
The change did not happen in all eligible words. It is hard to find a clear reason why some changed and others did not. Roughly, the more common a word the more likely that the change from flat /æ/ to broad /a:/ took place. It also looks as if monosyllables were more likely to change than polysyllables. Here are some examples to illustrate the variety:
There are some words in which both pronunciations are heard among southern speakers:
Evidence for the date of the shift comes from the Southern Hemisphere accents, those of Australia, New Zealand, and South Africa.
In Australian English, there is generally agreement with southern British in words like path, laugh, class. But before N+consonant, as in dance, plant, most Australians use a flat A (aunt and can't are exceptions and are invariably pronounced with /a:/). Phonetically the broad A is [a:]. In Australia there is variation in the word castle, both pronunciations being heard.
South African and New Zealand accents have a similar distribution of sounds to Australian. New Zealand accents are more like to use the broad version. Someone who knows them in detail could add more here.
The only areas of North America affected at all by the broad A are parts of New England and the Canadian Maritimes. In these areas, in the context where British speech has /a:/ or /A:/, both /æ/ and /A:/ tend to fall together into a new sound between the two. This may preserve the conditions that led to the change in British English. This version of broad A is strongly regionally marked and is not considered a prestige variety in North America, and the area in which it holds sway has tended to shrink.Variations
For speakers from the south of England, the As in all the above words are pretty consistently distinguished. The use of a broad A is one of the things that distinguish speakers in the south of England from more northerly speakers. However, broad As in the words half, calf, rather, aunt, can't and vase are more widespread in England.
Use of broad A in mass, lather is distinctly conservative and probably rare now. However, the other two fluctuations are both common. There are further complications here. While graph, telegraph, photograph can have either, graphic, graphology always have flat A. The broad A is more likely in /tra:ns/ (transfer, transport) than in /trænz/ (translate, trans-Atlantic).Southern Hemisphere accents