Characteristics
In broad transcription rhotics are usually symbolised as /r/ 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° for the alveolar approximant, a small capital R for the uvular trill, and a flipped small capital R for the voiced uvular fricative or approximant.
The fact that the sounds conventionally classified as "rhotics" vary greatly in both place and manner in terms of articulation, and also in their acoustic characteristics, 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 behaviour 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. The potential for variation within the class of rhotics makes them a popular area for research in sociolinguistics.
Read more about this topic: Rhotic Consonant