In order to facilitate the production of the harmonic notes on the clarinet, a small hole, closed by means of a key and called the "speaker," is bored near the mouthpiece. By means of this small hole the air-column is placed in communication with the external atmosphere, a ventral segment is formed, and the air-column divides into three equal parts, producing a triple number of vibrations resulting in the third note of the harmonic series, at an interval of a twelfth above the fundamental.[4] In a wind instrument with lateral holes the fundamental note corresponding to any particular hole is produced when all the holes below that hole are open and it itself and all above it are closed, the effective length of the resonating tube being shortened as each of the closed holes is successively uncovered. In order to obtain a complete chromatic scale on the clarinet at least eighteen holes are required. This series produces with the bell-note a succession of nineteen semitones, giving the range of a twelfth and known as the fundamental scale or _chalumeau_ register, so called, no doubt, because it was the compass (without chromatic semitones) of the more primitive predecessor of the clarinet, known as the _chalumeau_, which must not be confounded with the shawm or schalmey of the middle ages.
The fundamental scale of the modern clarinet in C extends from [Illustration]. The next octave and a half is obtained by opening the speaker key, whereby each of the fundamental notes is reproduced a twelfth higher; the bell-note thus jumps from E to B#, the first key gives instead of F its twelfth C#, and so on, extending the compass to [Illustration], which ends the natural compass of the instrument, although a skilful performer may obtain another octave by cross-fingering. The names of the holes and keys on the clarinet are derived not from the notes of the fundamental scale, but from the name of the twelfth produced by overblowing with the speaker key open; for instance, the first key near the bell is known not as the E key but as the B#. The use of the speaker key forms the greatest technical difficulty in learning to play the clarinet, on account of the thumb having to do double duty, closing one hole and raising the lever of the speaker key simultaneously. In a clarinet designed by Richard Carte this difficulty was ingeniously overcome by placing the left thumb-hole towards the front, and closing it by a thumb-lever or with a ring action by the first or second finger of the left hand, thus leaving the thumb free to work the speaker key alone.