The most common type of harmonica is the diatonic or Richter harmonica. It is typically used in blues, rock, country, and folk, but can be found in all styles of music. The diatonic harmonica is a single voice instrument and usually has 10 channels, each with one blow and one draw note. Due to their diatonic tonal layout, the player generally needs several instruments in different keys.
Tremolo harmonicas have double holes, each containing two reeds tuned to the same note, one tuned slightly higher than the other. Both reeds are either blow or draw. When they are played the slight difference in tuning creates a beautiful, vibrating or tremolo effect.