The alveolar trill is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents dental, alveolar, and postalveolartrills is [r], and the equivalent X-SAMPA symbol is r. It is informally and commonly called the rolling R or rolled R. Quite often, /r/ is used in phonemic transcriptions (especially those found in dictionaries) of languages like English and German that have rhotic consonants that are not an alveolar trill. This is partly due to ease of typesetting and partly because <r> is often the symbol used for the orthographies of such languages.
In the majority of Indo-European languages, this sound is at least occasionally allophonic with an alveolar tap[ɾ], particularly in unstressed positions. Exceptions to this include Spanish, Portuguese and Albanian, which treat them as separate phonemes.
There is a phone (different from [r]) which is exclusively used in Czech (in words such as rybáři 'fishermen'). Its manner of articulation is similar but the tongue is raised; it is partially fricative. It is orthographically represented by the letter <ř>, and in IPA as <r̝>. The basic manner of pronunciation is voiced but there is also a voiceless allophone [r̝̊].
Martínez-Celdrán, Eugenio; Ana Ma. Fernández-Planas & Josefina Carrera-Sabaté (2003), "Castilian Spanish", Journal of the International Phonetic Association33 (2): 255-259
Rogers, Derek & Luciana d'Arcangeli (2004), "Italian", Journal of the International Phonetic Association34 (1): 117-121
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Where symbols appear in pairs, the one to the right represents a voiced consonant. Shaded areas denote pulmonic articulations judged impossible.