":: This discussion belongs to years ago, so I hope that my comment will still be helpful. First of all, a necessary preamble: consonants in Italian can be double or single, as is well known; however, what is often overlooked in this description is that this contrast can only occur in certain positions, i.e. in between vowels or between a vowel and a ""semivowel"" (/w/ and /j/) or, for plosives, between a vowel and ""r""/""l""; in all other position all consonants have an indeterminate length. So in the word ""pasto"" (""meal"") neither the ""s"" nor the ""t"" can be described as being ""single"" or ""double"", they have an intermidiate, indeterminate length (pronouncing something like ""passto"" or ""pastto"" would be meaningless in Italian phonology, unlike other languages featuring geminate consonants like Finnish). So, applying this concept to the pronunciation of ""r"": single ""r"" is pronounced as a flap, double and indeterminate ""r"" are pronounced as a trill in standard pronounciation; for example ""caro"" (dear) contains a flap, while ""terra"" (earth) and ""treno"" (train) both contain a trill. You can check this listening to the entries of the ""Dizionario di Ortografia e Pronuncia"", published by the Italian state television RAI and which is (or used to be) available on this page. Using the trill in ""caro"", no matter how short, would inevitably turn it into ""carro"" (charriot). 
I am a native speaker, I've taught Italian to foreigners and I have studied and compared Italian phonology in depth. Of course, this claims are not sufficient for wikipedia, but there's an inherent problem when referencing publications on the subject. For long time phonology has been considered in Italy just a lesser part of grammar; more importantly, most study focussed on internal controversies and variations, i.e. describing the language from within the language, a more objective and scientific study of the phonetics is quite limited. It suffices to say that even IPA has hardly ever been used in the description of Italian phonetics, many authors using custom phonetic conventions even in promimenent works (such as the one I linked before). This means that while the contrast between /e/ and /ɛ/ or /ɔ/ and /o/ is widely discussed in many pubblication because there's a lot of variation in distribution, little work can be found about other distinctive traits of Italian phonology that are uniform or nearly so among native speakers (e.g. the assimilation of place of articulation of ""n"" with the following consonant; the lengthening of the vowel in stressed, open syllables inside words; phonetic value of syllables like in /pas.to/ contrasting with the prescribed orthographic split ""pa-sto"", syllable-timed rythm, etc.). The difference of distribution of flap vs trill for ""r"", as well as an accurate description of consonants of incontrastive length in general, has been largely disreguarded.   

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