Tremolo

Tremolo Harmonicas.
Folk musicians often use blues harmonicas to great effect, but for the playing of traditional Scottish and Irish music, the tremolo harmonica is the choice of many players. While similar in appearance to the regular blues harp, each hole has two blow and two draw reeds. Each pair of reeds is tuned to a particular note in the diatonic scale; however, one is tuned slightly sharp of its fellow. This creates the tremolo effect, and a tone that is particularly suited to jigs, reels, polkas and other traditional dance tunes.
Octave-tuned harmonicas are laid out in a similar fashion to the tremolo models, but the pairs of reeds are are tuned an octave apart, resulting in a full and strong tone.