TH fronting
Apparently, no accents with the merger completely merge the phonemes, because virtually all speakers of such accents know which words "should" have which sound; moreover, in many accents the two sounds appear in free variation. Where TH fronting is applied, pairs such as three/free, slither/sliver, and oath/oaf are homophonous.
The use of the labiodental fricatives /f/ and /v/ for the dental fricatives /T/ and /D/ is a well known feature of the proverbial Cockney. It has recently been noted as spreading through non-standard accents in England (cf. Trudgill 1988, 43). TH fronting is one of the boundaries between Estuary English and Cockney.
Although TH fronting 'pops up' occasionally in the middle and upper (middle) class English accents as well, there is still a marked social difference between working and middle class speakers. TH fronting can therefore serve as a 'boundary marker' between Cockney and Estuary English.
TH fronting is still a feature of Cockney which is extremely rare in the other social English accents. TH fronting can (still) serve as a 'boundary marker' between Estuary English and Cockney. Furthermore, TH fronting is currently making its way into the middle class English accent and thus into Estuary English. Whether it will creep into RP from there remains to be seen. One public school speaker has already been caught producing the labiodental instead of the dental fricatives. Whether this is an embryonic variant hailing a future sound change or a mere slip of the tongue, only the future can tell.
Here's a sample of a speaker of the Cockney accent that has th-fronting:
My dad came from Wapping and me mum came from Poplar. Me dad was one of 11 kids... and Wapping in them days really was one of the poorest parts of London. I mean they really didn't have shoes on their feet. I'm talking about 70 years ago now. Erm... and Poplar was... sli... just slightly a cut above Wapping; erm... you was either East End respectable or you was sort of East End villain, you know, and my family was respectable on both sides. But me father had a very tough time because his father died when he was 19, leaving him the only one working to bring up eleven brothers... 10 brothers and sisters and on a Thursday night he'd sometimes go home and the youngest two would be crying in the corner and he'd say "what's the matter with them, ma?" "Oh,well, Harry, you know it's Thursday night, and you don't get paid till tomorrow" and they literally didn't have any food in the house.
In that recording father and brother are pronounced and ; Thursday is pronounced .
