Retroflex Consonants
Retroflex consonants are a distinctive feature of Indian languages and Pashto, the language of the Pathan people of Afghanistan and Pakistan.
In producing the retroflex sound, the tongue is curled back in the mouth.
In English, although retroflex consonants are not represented by unique letters, nevertheless they exist plentifully. For example, the American pronunciation of the double T in bottle and the double D in fodder are retroflex.
The Japanese R is also retroflex.