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Types of Harmonicas

Harmonicas are commonly used in blues and folk music, but also in jazz, classical music, country music, rock and roll, and pop music. Increasingly, harmonicas are also finding their place in more electronically generated music, such as dance and hip-hop, as well as funk and acid jazz.
Diatonic (Blues) Harmonicas
The 10-hole diatonic harmonica is the most widely known type of harmonica. It has ten holes that offer the player a total of 19 notes (10 holes times a draw and a blow for each hole minus one repeated note) in a three-octave range. This is the harmonica commonly used in blues, country, and rock music, as well as by some skilled jazz players. The reeds of diatonic harmonicas produce the notes of the scale to which they are tuned. For example, a diatonic harmonica tuned to the key of C would produce the natural notes of the C scale without sharps and flats (picture the white keys on a piano, without the black keys). Each hole has two reeds; one plays when breath is exhaled (blow) and the other when inhaled (draw). The individual reeds are each tuned to play a different note on the scale. As with many other diatonic instruments, they come in all of the musical keys. One of the specialties of the 10-holed diatonic harmonica is its ability to play more than its basic 20 notes. It has the ability to produce 42 notes, (including 4 repeats), ending up with a complete 3 chromatic octave range, plus two extra half-steps on the high end. This requires the use of special techniques such as bending and overblowing. Furthermore, these techniques are used to produce many different effects. The most common is slurring (linking from a regular note in the scale to an overblow or overdraw) to the bent note, or playing straight into the note.
Tremolo Harmonicas
The tremolo harmonica's distinguishing feature is that it has two reeds per note, with one a bit sharp and the other a bit flat. This provides a unique wavering or warbling sound created by the two reeds being slightly out of tune with each other and the difference in their subsequent waveforms interacting with each other.
Octave Harmonicas
Octave harmonicas have two reeds per hole which are tuned to the same note a perfect octave apart. Many share their basic design with the tremolo harmonica.
Chromatic Harmonicas
The chromatic harmonica usually uses a button-activated sliding bar to redirect air from the hole in the mouthpiece to the selected reed-plate. This harp can be used for any style - Celtic, classical, jazz, blues (commonly in third position) - as well as many other styles.
Source: Wikipedia