Quote:

Many players are under the impression that it is necessary to use lip pressure to make high notes on the clarinet; while in fact the less pressure used the easier the high notes respond. To prove this play this G Sustain it for two beats and, without changing the embouchure, turn the index finger downward so that it uncovers half the hole and the high E will come out, not only with great facility but with nice quality and smoothness of tone.

This partial uncovering of the hole has many advantages and should be patiently studied, for it enables one to slur from a lower interval to many of the high notes with the greatest ease and as softly as you wish.