There are mainly two ways of using the corners. One can play with nonmovable (locked) corners or movable corners.

If playing with locked corners, lip compression will come mainly from the pinching power of the lips. The aperture will get flatter as one ascends, yielding a thinner tone quality. At some point the lips will thin out, a bit similar to the smile embouchure.
Nonmovable corners works well with tight corners.

Playing with movable corners, there is need for the orbicularis oris to flex. This flexing is what moves the corners, not the corner muscles. Tight corners will inhibit this movement. Playing this way, the aperture can maintain the shape, balancing the pinching and puckering forces. Tone quality will then be the same in all registers.

When learning to play with movable corners, one should feel an inward motion of the corners when ascending. Lip slurs and octave glissandi are good for this purpose. One might also feel that the embouchure moves a bit forward.
Descending, the embouchure should relax. What one should not do, is relaxing air compression while slurring downwards.

Octave glizzandi
This exercise is found both in the Shew, Vizzutti, Reinhart and Stevens books.

I have tried to do a couple of demonstrations of what moving corners look like. The movment is actually very small.
Octave glizzandi, closeup
Octave glizzandi on lips, closeup

To see more about what moving corners look like, get Bill Carmichael's video.

Copyright (c) Rune Aleksandersen 1998 - 2002