In phonetics, rhotic consonants, or "R-like" sounds, are liquid consonants that are traditionally represented orthographically by symbols derived from the Greek letter rho, including , in the Latin script and , in the Cyrillic script. They are transcribed in the International Phonetic Alphabet by upper- or lower-case variants of Roman , : , , , , , , , and as well as by the lower-case turned Roman (the symbol for the near-open central vowel) combined with the "non-syllabic" diacritic, that is . This class of sounds is difficult to characterise phonetically; from a phonetic standpoint, there is no single articulatory correlate (manner or place) common to rhotic consonants. Rhotics have instead been found to carry out similar phonological functions or to have certain similar phonological features across different languages. Being "R-like" is an elusive and ambiguous concept phonetically and the same sounds that function as rhotics in some systems may pattern with fricatives, semivowels or even stops in others. For example, the alveolar flap is a rhotic consonant in many languages, but in North American English, the alveolar tap is an allophone of the stop phoneme , as in water. It is likely that rhotics are not a phonetically-natural class but a phonological class. Some languages have rhotic and non-rhotic varieties, which differ in the incidence of rhotic consonants. In non-rhotic accents of English, /ɹ/ is not pronounced unless it is followed directly by a vowel. Types The most typical rhotic sounds found in the world's languages are the following: Trill (popularly known as rolled r): The airstream is interrupted several times as one of the organs of speech (usually the tip of the tongue or the uvula) vibrates, closing and opening the air passage. If a trill is made with the tip of the tongue against the upper gum, it is called an apical (tongue-tip) alveolar trill; the IPA symbol for this sound is . Most non-alveolar trills, such as the bilabial one, however, are not considered rhotic. Many languages, such as Bulgarian, Swedish, Norwegian, Frisian, Italian, Spanish, Russian, Polish, Ukrainian, Dutch and most Occitan variants, use trilled rhotics. In the English-speaking world, the stereotyped Scottish rolled is well known. The "stage pronunciation" of German specifies the alveolar trill for clarity. Rare kinds of trills include Czech (fricative trill) and Welsh (voiceless trill). The uvular trill is another kind of rhotic trill; see below for more. Tap or