The harmonica is a wind instrument using reeds of different lengths inside an oblong metal case, arranged from left to right in order of the notes of the major scale. The scale is played by alternately blowing and sucking, as the tonic, mediant and dominant notes are blown, whereas the super-tonic, sub-dominant, sub-mediant and leading notes are sucked. Harmonicas are made in several keys, especially C, G and F major.  More advanced models, called chromatic harmonicas,  have a lever on the right side, which changes the plate to a new set of reeds tuned one semi-tone higher. This enables the full chromatic scale to be played.

The harmonica is a very compact and lightweight instrument, easily carried in a pocket. It is popular for use on camping trips, long group outings in buses, and with hikers, where one player can accompany the singing of the rest of the group.  It is also used by some rock guitar groups and Rhythm and Blues musicians. A characteristic wowing effect, or modulation, can be achieved by cupping the right hand over the outlets, and gently waving the fingers to vary the flow of air.