Th-debuccalization is a process in varieties of Scots and Scottish English where a voiceless dental fricative /θ/ (spelled th) at the beginning of a word and between vowels becomes the voiceless glottal fricative [h]. It is a stage in the process of lenition.
Th-debuccalization occurs mainly in Glasgow and across the Central Belt. A common example is [hiŋk] for think. This feature is becoming more common in these places over time, but is still variable. In word final position, [θ] is used, as in standard English.
The existence of local [h] for /θ/ in Glasgow complicates the process of th-fronting there, a process which gives /f/ for historical /θ/. Unlike in the other dialects with th-fronting, where /f/ solely competes with /θ/, in Glasgow, the introduction of th-fronting there creates a three-way variant system of [h], [f] and [θ].
Use of [θ] marks the local educated norms (the regional standard), while use of [h] and [f] instead mark the local non-standard norms. [h] is well-known in Glasgow as a vernacular variant of /θ/ when it occurs word-initially and intervocalically, while [f] has only recently risen above the level of social consciousness.
Given that th-fronting is a relatively recent innovation in Glasgow, it was expected that linguists might find evidence for lexical diffusion for [f] and the results found from Glasgow speakers confirm this. The existing and particular lexical distribution of th-debuccalization imposes special constraints on the progress of th-fronting in Glasgow.
In accents with th-debuccalization, the cluster /θr/ becomes [hr] giving these dialects a consonant cluster that doesn't occur in other dialects. The replacement of /θr/ with [hr] leads to pronunciations like: