This clarinet consists of a thin-walled plastic tube into which a clarinet mouthpiece can be inserted at one end. (A real clarinet is available for comparison.) About one-third of the length of the tube from the mouthpiece end is a hole (indicated by the red arrow, about one inch below the thumb in the photograph at the right) which represents the register hole of the clarinet. Closing the register hole produces a low tone, the fundamental. Opening the register hole produces the third harmonic, a note a musical twelfth (one octave plus a fifth) above the fundamental.
In an actual clarinet, opening and closing the register hole with your left thumb changes the clarinet register from the Chalameau, or lower register to the clarion, or middle register. Covering or opening the finger holes with your remaining fingers provides the notes of the scale within each register.