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Harmonicas are hand-held instruments that create sound when you blow or suck air through small reeds. In the family of mouth organs, also known as mouth harps, harmonicas are generally seen as a folk instrument because they are accessible to an untrained musician and produce raw, emotional music. In many cultures, instruments like harmonicas have been made of glass, rock, wood, or other materials, but most commonly the body is metal or plastic and the reeds are metal.
Harmonicas are one of the few known instruments that can be played both by "blowing" (breathing out) and "drawing" (breathing in) with strikingly different results. We categorize harmonicas as free reed instruments because they remain stationary during play. Unlike an instrument like a trumpet or saxophone, the harmonica is played with the lips, tongue, and breathing patterns exclusively, with no other buttons or keys to move or bend the air to vary the pitch.
In classification of musical instruments, harmonicas are also known as idiophones. This is because the reeds create the sound from their unique resonating characteristics, as opposed to a string or a membranous drum. Reeds aren't tuned by people the way that the tautness of strings can be affect tone. Rather, a difference in the sound of harmonicas comes from selecting the right reeds and varying the pressure or form of the air blown across them.
Several pre-tuned reeds, around a dozen, are suspended in narrow chambers in the body of harmonicas, known as the comb. Inhaling or exhaling from one side of the chamber vibrates one or a few reeds at a time to create interesting textural or harmonic sounds coming out the other side. The number of holes, and therefore reeds, can vary from four on a child's harmonica to sixteen for a professional musician. Square holes funnel air to two possible reeds of a pair, known as the "blow" and "draw" reed, to produce different sounds.
One familiar kind of tuning for harmonicas is called diatonic. This means that the reeds play the notes of a major scale without any flat or sharp notes relative to the key note. The most common are a type of diatonic harmonica where each note is played by a single reed, although other types use the combination of two reeds to create a major note.
In American music history, harmonicas are associated with folk traditions of blues, bluegrass, and country. These communal forms of music are designed to be practiced spontaneously anywhere people congregate. The harmonica became popular for these events because it is lightweight, inexpensive, simple to learn proficiently, and creates evocative music.