A phonological characteristic. It refers to the positioning of the tongue; the middle of the tongue arches to touch the top of the mouth. Compare this to standard r, like in French "vraiment," where the r is
articulated at the front of the mouth using the tip of the tongue. Retroflexes usually appear at the end of a word. Standard American English uses retroflexes (Boston accents don't), standard British English doesn't, Putonghua Mandarin Chinese doesn't, Beijing dialect does.