Rhotics, or "R"-like sounds, are non-lateral liquids. This class of sounds is difficult to characterise phonetically, though most of them share some acoustic peculiarities, most notably a lowered third formant in their sound spectrum. However, "being r-like" is a strangely elusive feature, and the very same sounds that function as rhotics in some systems may pattern with fricatives, semivowels or even stops in others. The most typical rhotic sounds found in the worlds 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, we speak of an apical (tongue-tip) alveolar trill. If it is made with the uvula against the back of the tongue, we speak of a uvular trill.
- Tap or flap (these terms refer to very similar articulations): Not unlike a trill, but involving just one brief interruption of airflow. In many languages taps are used as reduced variants of trills, especially in fast speech. Note, however, that in Spanish, for example, taps and trills contrast, as in pero ("but") versus perro ("dog"). In American English flaps do not function as rhotics but are realisations of intervocalic apical stops ([t] or [d], e.g. in city or butter).
- Alveolar or retroflex approximant, as in most accents of English (with minute differences): The front part of the tongue approaches the upper gum, or the tongue-tip is curled back towards the roof of the mouth ("retroflexion"). No or little friction can be heard, and there is no momentary closure of the vocal tract.
- Uvular or velar approximant or fricative: The back of the tongue approaches the soft palate or the uvula. Standard French, German or Danish [r]'s are variants of this type of rhotic. If fricative, the sound is often impressionistically described as harsh or grating.
See also: rhotic (accents of English).