The list of manners of articulation is clearly insufficient to account for all the linguistic contrasts that occur. But it is useful in that it allows us to begin with a simple model which distinguishes between only three degrees of obstruction to the airstream (complete stoppage, restricted flow, and unimpeded flow; here called stop, fricative, and approximant) and three other gestures (trill, tap, and flap), which occur at a more limited number of places of articulation.