R-labialization

Pronunciation of the phoneme /r/ in the English language has many variations in different dialects.

Depending on dialect, /r/ has at least the following allophones in varieties of English around the world:

In most dialects /r/ is labialized [ɹ̠ʷ] in many positions, as in reed [ɹʷiːd] and tree [tɹ̥ʷiː]; in the latter case, the /t/ may be slightly labialized as well. In General American, it is labialized at the beginning of a word but not at the end.

/r/ is commonly pronounced as a voiceless dental flap [ɾ̪̊] in the cluster /θr/ (spelled thr), even in dialects wherein the usual allophone of the /r/ phoneme in other phonetic environments is an approximant; for example as in red (/rɛd/ → [ɻʷɛd]), dread (/drɛd/ → [dɻʷɛd~d͡ʒɻʷɛd]) and tread (/trɛd/ → [tʰɻ̊ʷɛd~t͡ʃɻ̊ʷɛd]), but not thread (/θrɛd/ → [θɾ̪̊ɛd]). This results from the process of assimilation: the /r/ becomes voiceless and dental as a result of the preceding voiceless dental fricative /θ/.

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