The mouthpiece is an item in woodwind and brass instruments that is directly manipulated by the mouth parts and the respiratory system of an instrumentalist. Its primary importance is to transmit vibrations from one of three sources: a column of air blown directly into a mouthpiece, a reed or the instrumentalist's own lips. These vibrations are then transmitted into the rest of the instrument for further modification that would ultimately determine the timbre of the instrument and the pitches that it is capable of playing. Flutes and reed instruments fall into the woodwind category, while trumpets, trombones, tubas and similar instruments fall into the brass category.
Mouthpieces can be made out of different materials -- metal, wood or plastic -- and their design depends upon on the mechanism by which they sound. The pure metal mouthpiece is standard for all orchestral flutes and all orchestral brass instruments. Modern single-reed instruments like the clarinet and saxophone usually have rubber mouthpieces combined with a reed and metal ligatures that fix them into place. The double-reed instruments such as oboes and bassoons have mouthpieces consisting of two reeds tied to metal tubing.
The flute mouthpiece, at its most basic, is nothing more than a cylindrical piece of metal with a blowhole, an open end that connects to the rest of the instrument and another end closed shut by a piece of metal. The player uses the flute mouthpiece to cause a jet of air to strike against the edge of the blowhole to produce a sound. This is accomplished by forming his lips in a certain manner (embouchure) and blowing across the blowhole in a similar manner to blowing across the opening of a bottle.
The brass mouthpiece is a cup or a bell-shaped piece of metal, the wider end of which receives the player's lips and the narrower end connects to the rest of the instrument. The player uses the brass mouthpiece to transmit the vibrations of his own lips towards the instrument to produce a sound. This is done by forming the proper embouchure to control airflow and vibration frequency.
The harmonica is a mouthpiece by itself. Most professional harmonicas are made out of metal. The player uses the harmonica "mouthpiece" only as a means to blow air towards its comb that houses its free reeds.