When Matthias Hohner began manufacturing small "mouth organs" in Germany in 1857, improving on a 'free reed' instrument designed by Christian Buschmann in 1821, he had no idea how popular they would become. Inspired by the mouth organs of Asia, the Western harmonica rose to prominence in a variety of styles from "classical" concert repertoire to folk music and 'the blues'.
The sounds of the harmonica are produced by a series of small pipes, each containing a reed which vibrates when you inhale or exhale through the pipes. You stop pitches you don't want sounded with your tongue.
There are two basic types of harmonica: diatonic and chromatic. Diatonic harmonicas can only sound pitches for one scale. For example, a C MAJOR diatonic harmonica only produces the tones of the C MAJOR scale. Chromatic harmonicas can sound every pitch within their range when you push a button on the side of the instrument.
The harmonica has been called "the characteristic instrument of the deepest blues", popular among blues musicians in part because it is relatively inexpensive and very portable. It played a prominent role in the small blues bands of the southern United States in the decades surrounding World War II. Among the many gifted blues harmonica players was John Lee (Sonny Boy) Williamson, who, having learned to play harmonica in Memphis, made it a main instrument in his band, equal to the guitar. Williamson later experimented with electric amplification for his harmonica, inspiring future generations of blues musicians .
To those familiar with the sound of the blues, it may seem surprising that blues musicians might use a diatonic harmonica. How do they achieve those striking "blue notes" - that is, those chromatic pitches which seem to clash with the underlying harmonic structure? Easy--they use a harmonica that's tuned to a different key than that of piece they are playing.
A related invention created in 1959 is the melodica, familiar to many music educators. With this instrument, the reeds are controlled by buttons or a piano-like keyboard. Another extremely popular instrument which makes use of a piano keyboard and reeds to produce harmonica-like sounds is the accordion. Instead of blowing into pipes to make the reeds sound, you squeeze a bellows to pump air into them.