Looking at the IPA, many different types of sounds are given symbols based of of the Latin R,r: approximants, trills, taps/flaps; both coronal and uvular segments.
Sometimes, these sounds are historically related, for example the French uvular approximant /ʁ/ replaced the earlier alveolar trill /r/.
My question is, why do these sounds pattern together despite being so phonetically different? Why do they often historically interchange and why do foreign speakers perceive foreign rhotics as comparable to their own, even if they are vastly different in terms of articulation?
r
sounds sometimes pattern partly like vowels as in at least Croatian and Slovenian. Also voiced velar fricatives might be anr
in some languages but not in others such as Arabicغ
and Georgianღ
, both of which have another sound which functions as anr
.z
andʒ
can also pattern liker
or even alternate with them. So I'd agree with @Askalon below that it's hard to characterize segments as "rhotic" on a purely phonetic basis.