Th-debuccalization

In English, the digraph ⟨th⟩ represents in most cases one of two different phonemes: the voiced dental fricative /ð/ (as in this) and the voiceless dental fricative /θ/ (thing). More rarely, it can stand for /t/ (Thailand, Thames) or, in some dialects, even the cluster /tθ/ (eighth). It can also be a sequence rather than a digraph, as in the /t.h/ of lighthouse.

In standard English, the phonetic realization of the dental fricative phonemes shows less variation than for many other English consonants. Both are pronounced either interdentally, with the blade of the tongue resting against the lower part of the back of the upper teeth and the tip protruding slightly or alternatively with the tip of the tongue against the back of the upper teeth. The interdental position might also be described as "apico-" or "lamino-dental". These two positions may be free variants, but for some speakers they are complementary allophones, the position behind the teeth being used when the dental fricative stands in proximity to an alveolar fricative, as in clothes (/ðz/) or myths (/θs/). Lip configuration may vary depending on phonetic context. The vocal folds are abducted. The velopharyngeal port is closed. Air forced between tongue surface and cutting edge of the upper teeth (interdental) or inside surface of the teeth (dental) creates audible frictional turbulence.

The difference between /θ/ and /ð/ is normally described as a voiceless-voiced contrast, as this is the aspect native speakers are most aware of. However, the two phonemes are also distinguished by other phonetic markers. There is a difference of energy (see: Fortis and lenis), the fortis /θ/ being pronounced with more muscular tension than the lenis /ð/. Also, /θ/ is more strongly aspirated than /ð/, as can be demonstrated by holding a hand a few centimeters in front of the mouth and noticing the differing force of the puff of air created by the articulatory process.

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