a harmonica is small musical instrument that is played with the mouth by blowing into holes in its side. harmonicas are cheap and easy to play. harmonicas produce their musical sounds from the vibrations of reeds in the harmonica's metal case. harmonicas are used in blues music, folk music, rock and roll music, and pop music. a special type of harmonica, the chromatic harmonica, is used in jazz and classical music. harmonicas are made in several different keys: g, a♭, a, b♭, b, c, d♭, d, e♭, e, f, and f♯. each key can play a different range of notes.