Rhotic consonant
Rhotic consonants, or "R"-like sounds, are non-lateral liquid consonants.  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 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 Template:IPA. If it is made with the uvula against the back of the tongue, it is a uvular trill; the IPA symbol for this sound is Template:IPA. The bilabial trill, however, is not considered a rhotic. Many languages, for example Russian, Italian or Spanish, use trilled rhotics. In the English-speaking world, the stereotyped (if not actually very common) Scottish rolled  is well-known. The "stage pronunciation" of German specifies the alveolar trill for clarity. Rare kinds of trills include Czech r Template:IPA (fricative trill) and Welsh rh Template:IPA (voiceless 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 Template:IPA ("but") versus perro Template:IPA ("dog"). In some English dialects (for example American, Australian) flaps do not function as rhotics but are realizations of intervocalic apical stops (Template:IPA or Template:IPA, for example in rider or butter). The IPA symbol for this sound is Template:IPA.
- 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. The IPA symbol for the alveolar approximant is Template:IPA and the symbol for the retroflex approximant is Template:IPA. It is to be noted that there is a distinction between an unrounded retroflex approximant and a rounded variety to be found in Anglo-Saxon and even to this day in some dialects of English, where the orthographic key is r for the unrounded version and usually wr for the rounded version (these dialects will make a differentiation between rite/right and write/wright).
- Uvular or velar approximant or fricative: The back of the tongue approaches the soft palate or the uvula. The standard Template:IPA's in French, German, and Danish are variants of this type of rhotic. If fricative, the sound is often impressionistically described as harsh or grating. This includes the voiced uvular fricative, voiceless uvular fricative, voiced velar fricative, voiceless velar fricative, and the velar approximant. In northern England, there used to be accents which employed the voiced velar fricative, which was called a "burr." In southern England, the velar approximant is considered a prestigious kind of lisp, though it does not occur in many other national dialects.
In broad transcription rhotics are usually symbolised as Template:IPA unless there are two or more types of rhotic in the same language.  The IPA has a full set of different symbols which can be used whenever more phonetic precision is required: an r rotated 180 Template:IPA for the alveolar approximant, a small capital R Template:IPA for the uvular trill, and a flipped small capital R Template:IPA for the voiced uvular fricative.
The fact that the sounds conventionally classified as "rhotics" vary greatly in both place and manner of articulation has led several linguists to investigate what, if anything, they have in common that justifies grouping them together. One suggestion that has been made is that each member of the class of rhotics shares certain properties with other members of the class, but not necessarily the same properties with all; in this case, rhotics have a "family resemblance" with each other rather than a strict set of shared properties. Another suggestion is that rhotics are defined by their behavior on the sonority hierarchy, namely, that a rhotic is any sound that patterns as being more sonorous than a lateral consonant but less sonorous than a vowel.