Th-fronting

# Background
Th-fronting is a merger that occurs (historically independently) in Cockney, Newfoundland English, African American Vernacular English, and Liberian English (though the details differ among those accents), by which Early Modern English Template:IPA merge with Template:IPA. 
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. Th-fronting of the voiced consonant Template:IPA does not occur at the beginning of a word, but only intervocalically and at the end of a word.
The use of the labiodental fricatives Template:IPA and Template:IPA for the dental fricatives Template:IPA and Template:IPA 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).
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 is regarded as a 'boundary marker' between Cockney and Estuary English, as depicted in the first descriptions of the latter form of English   and confirmed by a phonetic study conducted by researcher Ulrike Altendorf. Nevertheless, Altendorf points out that th-fronting "pops up" occasionally in middle class (Estuary) speech as well and concludes that "it is currently making its way into the middle class English accent and thus into Estuary English". 
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 eleven 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 seventy 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 nineteen, leaving him the only one working to bring up eleven brothers... ten 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 Template:IPA and Template:IPA; Thursday is pronounced Template:IPA.
Th-fronting has been spreading in Southern England at a slower rate than t-glottalization.
Th-fronting in the speech of working-class adolescents in Glasgow was reported in 1998 and when it was reported, it provoked interest which was both academic interest and general interest.
The finding of th-fronting in Glaswegian creates a difficulty for models of language change which hinge on dialect contact associated with geographical mobility since the Glaswegian
speakers who used  most in the 1997 sample are also those with the lowest geographical mobility.
See also
- List of th-fronting homophones
- Speech_sound_disorder