The R Sound in Modern English

The sounds that we associate with the letters k and g are produced in the back of the mouth by briefly closing the vocal tract there. The sounds of other letters are made in a similar way by blocking and releasing the air flow in the front part of the mouth. But then we have this separate group of sounds that are somewhere in the middle. There is some type of constriction going on, but the constriction is so subtle or so weak, that the tongue is actually doing much of the work to shape the sound. In that regard, these sounds are sort of like vowels, since the constricting is subtle and slight. And that's why linguists call them approximates.