Mouthpieces are human-machine interfaces designed to transmit the maximum musical information from the mouth, lips, tongue and lungs of the performer into the air column of the instrument. On the performer's side, mouthpiece must be matched to embouchure, the exact way of setting the mouth parts to transmit the maximum of musical information to the mouthpiece.
Thus, the mouthpiece, attached to its instrument, is a means of extending the expressive power of the upper body. The typical mouthpiece is capable of playing only a few notes on its own, often with poor tone. By varying the air column in a myriad of ingenious ways, these few notes are transformed into many notes of many tones on the various instruments.
Trumpet mouthpiece from the side
In western instruments of the European classical tradition, there are several different kinds of mouthpieces. One of the most common is the one seen on brass instruments, which is a type also used on several non-western instruments. This consists of a simple circular opening which leads to the main body of the instrument. The player causes his lips to vibrate while they are placed next to this opening. This causes the column of air contained within the instrument to vibrate.
Trumpet mouthpiece from the front.
Note the cushioning effect of the rim, the cup in which the lips vibrate, and the small aperture which goes through into the rest of the instrument
The mouthpiece on single-reed instruments, such as the clarinet is quite different. These mouthpieces are basically wedge-shaped, with the reed being placed against the flat surface closest to the player's bottom lip. Here, the player causes the reed to vibrate, which in turn sets the column of air contained within the instrument in vibration. Near the top of the mouthpiece there is a small opening into the inside of the instrument. As with the brass instruments, the shape of the bore immediately beyond the opening can greatly affect the sound of the instrument. On some woodwind instruments, the bore here is a simple extension of the main body of the instrument, although it is possible to widen it by various means.