A harmonica that has two reeds in each hole, which vibrate, creating pitched tones when air is blown or drawn through the harmonica. There are two types of double-reed harmonicas.
Tremolo tuned: the two reeds are tuned slightly different from one another, resulting in a chorus/vibrato/tremolo effect. This provides a rich tone that is popular for ballads, Latin music, reggae, folk, and gospel.
Octave tuned: this is the harmonica equivalent of a 12-string guitar, where the two reeds are tuned an octave apart. Octave harmonicas are popular in Cajun, old time, Celtic, and traditional music.
See also Single Reed Harmonica