Musical wind instruments or aerophones produce their sound by setting a body of air into vibration. Musicologists distinguish between two main groups of aerophones, the pipe aerophones (flute, saxophone, trumpet etc,) and the free aerophones (accordion, mouth organ, organ etc.)
Pipe aerophones typically consist of a resonance body (commonly tube shaped) where the air is brought to resonate by the player blowing into or over an opening. The pitch depends on the length of the vibrating column of air, which is be manipulated in numerous ways: By opening and closing a series of holes in the resonance body (flutes), by opening and closing valves that increase the tube length and lower the pitch (trumpets) or by using a sliding mechanism to lengthen and shorten the tube (trombone). In addition, the strength by which the player blows into the instrument can make the air vibrate at different pitches (harmonics), a strategy relied upon by the clarinet and the trumpet for example.
Pipe aerophones are often divided into two general groups, brass instruments and woodwind instruments, where the difference is not made the by material used for the instrument but the way in which it is played. Players of brass instruments make their lips vibrate while blowing (trumpet etc.) while players of woodwind instruments don't (flute, clarinet etc.). Consequently, the saxophone belongs to the 'woodwind' group of instruments although it is made of brass while the wooden didjeridu belongs to the 'brass' group. Hence, some authors propose the term lip vibrated aerophones for 'brass instruments'. Woodwind instruments are further subdivided into flutes or edge instruments, where the player blows across a hole (traverse flute) or against an edge (recorder flute, whistle) and reeds or reedpipes, where the air is set to vibrate by blowing across a reed (oboe, saxophone etc).
In free aerophones the pitch of the sound is not determined by the length of the vibrating air column. The simplest type of a free aerophone is the sizzling sound of a whip. Most free aerophoes belong to the subfamily of free reed instruments or free reeds where the pitch is controlled by changing the length and the thickness of a reed. Well known free reed instruments are the accordion, the harmonica or the harmonium. Free reed instruments may also use pipes, like the Chinese sheng where, however, the pipes act merely to reinforce the sound and their length has no effect on the pitch.