The most common sound represented by the letter r in English is the postalveolar approximant, pronounced a little more back and transcribed more precisely in IPA as ⟨ɹ̠⟩. For ease of typesetting, English phonemic transcriptions might use the symbol ⟨r⟩ instead of ⟨ɹ⟩, even though the former symbol represents the alveolar trill in phonetic transcription.
Allophone of [ɾ ~ ʁ] in recent loanwords, used even by many speakers who do not use coda alveolar, post-alveolar or retroflex approximants in native words. Varies individually. Generally not as onset or final e.g. trailer[ˈtɾejle̞ʁ].
Some or all coda /ɾ/ may instead be a guttural or an approximant, varying by speaker, albeit more likely to be aspirated or deleted altogether than to be an approximant among native speakers when as final. See Portuguese phonology.
Allophone of [ɾ ~ ʁ] in the syllable coda. May also be retroflex, post-alveolar and/or rhotic vowel. As most rhotic coda in colloquial Brazilian Portuguese, likely to get deleted as final or at the end of sentences
Browman, C.P.; Goldstein, L. (1995), "Gestural syllable position in American English", in Bell-Berti, F.; Raphael, L.J., Producing Speech: Contemporary Issues: for Katherine Safford Harris, New York: AIP, pp. 9–33
Cornyn, William (1944), Outline of Burmese Grammar, Supplement to Language, vol. 20 no. 4, Baltimore: Linguistic Society of America
Delattre, P.; Freeman, D.C. (1968), "A dialect study of American R's by x-ray motion picture", Linguistics44: 29–68
Engstrand, Olle (1999), "Swedish", Handbook of the International Phonetic Association, Cambridge University Press, pp. 140–142, ISBN9780521637510
Fougeron, C (1999), "Prosodically conditioned articulatory variation: A Review", UCLA Working Papers in Phonetics97, pp. 1–73
Grønnum, Nina (2003), Why are the Danes so hard to understand?
Hallé, Pierre A.; Best, Catherine T.; Levitt, Andrea (1999), "Phonetic vs. phonological influences on French listeners' perception of American English approximants", Journal of Phonetics27 (3): 281–306, doi:10.1006/jpho.1999.0097
Ikekeonwu, Clara I. (1999), "Igbo", Handbook of the International Phonetic Association, Cambridge University Press, pp. 108–110, ISBN9780521637510
Kohler, Klaus (1995), Einführung in die Phonetik des Deutschen, Berlin: Erich Schmidt Verlag