The class of rhotics is subject to extensive variation, and a reliable phonetic correlate has not been found. This variation is also why identifying a segment as a rhotic in an unknown language is not a trivial matter. In contrast to other phonological classes whose membership is attributed based on principled criteria, the set of rhotics is arbitrary. This article identifies two properties independent of phonetics which characterize rhotics cross-linguistically PROCEDURAL STABILITY—rhotics that are implicated in phonological processes can vary in a phonetically arbitrary manner without perturbing the process itself—and DIACHRONIC STABILITY: the phonetics of rhotics can vary in diachronic evolution without impact on their phonotactics. On the empirical side the article establishes a cross-linguistic survey of the phonetic variability of rhotics. It is also argued that the phonetic realization of a rhotic may be unpredictable and divorced from its phonological identity and this shows that languages are happy to instantiate an arbitrary phonetics-phonology relationship. Finally, it is argued that rhotics show that the interface which maps phonological objects to their phonetic instantiations is capable of handling an arbitrary relationship. Further, there is no reason to assume that this property of the interface is specific to rhotics; in principle, all phonetic and phonological categories could enter into an arbitrary relationship. This has important implications for theories which seek to impose phonetic or naturalness based constraints on phonology: it is difficult to see how the relationship between a phonetic object which has no obvious articulatory connection to its phonological representation could be considered phonetically natural. Rhotics thus provide support for the view of substance-free phonology whereby phonological objects are devoid of any reference to phonetic categories.