Woodwinds
In the case of the clarinet, the instrument's single reed beats against its mouthpiece, opening and closing the instrument's cylindrical closed tube to produce a tone. When the instrument is overblown, with or without the aid of its register key, the pitch is a twelfth higher.
In the case of a saxophone, which has a similar mouthpiece-reed combination to the clarinet, or of an oboe, where double reeds beat against each other to the same effect, the conical-shaped bore of these instruments gives their the closed tube properties of an open tube; when overblown, the pitch jumps an octave higher.
Read more about this topic: Overblowing