The harmonica, also known as a French harp, mouth harp or mouth organ, is a free reed wind instrument used worldwide in many musical genres, notably in blues, American folk music, classical music, jazz, country, and rock. The many types of harmonica include diatonic, chromatic, tremolo, octave, orchestral, and bass versions. A harmonica is played by using the mouth to direct air into or out of one holes along a mouthpiece. Behind each hole is a chamber containing at least one reed. The most common is the diatonic Richter-tuned with ten air passages and twenty reeds, often called the blues harp. A harmonica reed is a flat, elongated spring typically made of brass, stainless steel, or bronze, which is secured at one end over a slot that serves as an airway. When the free end is made to vibrate by the player's air, it alternately blocks and unblocks the airway to produce sound.

The Richter-tuned harmonica, or 10-hole harmonica or blues harp, is the most widely known type of harmonica. It is a variety of diatonic harmonica, with ten holes which offer the player 19 notes in a three-octave range.