In most dialects /r/ is labialized  in many positions, as in reed [i:d] and tree [ti:]; 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.
In many dialects, /r/ in the cluster /dr/, as in dream, is realized as a postalveolar fricative  or less commonly alveolar . In /tr/, as in tree, it is a voiceless postalveolar fricative [?] or less commonly alveolar . In England, while the approximant has become the most common realization, /r/ may still be pronounced as a voiceless tap  after /?/ (as in thread).
There are two primary articulations of the approximant /r/: apical (with the tip of the tongue approaching the alveolar ridge or even curled back slightly) and domal (with a centralized bunching of the tongue known as "molar r" or sometimes "bunched r" or "braced r"). Peter Ladefoged wrote: "Many BBC English speakers have the tip of the tongue raised towards the roof of the mouth in the general location of the alveolar ridge, but many American English speakers simply bunch the body of the tongue up so that it is hard to say where the articulation is". The extension to the IPA recommends the use of the IPA diacritics for "apical" and "centralized", as in ⟨, ⟩, to distinguish apical and domal articulations in transcription. However, this distinction has little or no perceptual consequence, and may vary idiosyncratically between individuals.
All English accents around the world are frequently characterized as either rhotic or non-rhotic. The majority of accents in England, Wales, Australia, New Zealand, and South Africa speak non-rhotic accents, and in these dialects the historical English phoneme /r/ is not pronounced except when followed by a vowel. However, the historical /r/ is pronounced in all contexts in rhotic accents, which are spoken across the majority in Scotland, Ireland, the United States, and Canada. Thus, a rhotic accent pronounces marker as ['m?rk?r], while a non-rhotic accent pronounces the same word as ['m?:k?]. Generally speaking in rhotic accents, when /r/ is not followed by a vowel phoneme, it surfaces as r-coloring of the preceding vowel or its coda: nurse [n?s], butter ['b?t?].
R-labialization, not to be confused with the rounding of initial /r/ described above, is a process occurring in certain dialects of English, particularly some varieties of Cockney, in which the /r/ phoneme is realized as a labiodental approximant [?] in contrast to an alveolar approximant [?]. To English-speakers not used to [?], it is nearly indistinguishable from /w/.
Use of labiodental /r/ is commonly stigmatized by prescriptivists. Regardless, it is used in many other languages and its use is growing in many accents of British English. Most speakers who do so are from the southeast of England, particularly London. It is also occasionally heard in some speakers of the Boston accent but more often in an exaggerated parody of those dialects.
R-labialization leads to pronunciations such as the following:
However, replacement of /r/ by some kind of labial approximant may also occur as symptom of a speech defect called rhotacism or, more precisely, derhotacization.