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. Reeds are pre-tuned to individual pitches. Tuning may involve changing a reeds length, the weight near its free end, or the stiffness near its fixed end. Longer, heavier and springier reeds produce deeper, lower sounds; shorter, lighter and stiffer reeds make higher- pitched sounds. If, as on most modern harmonicas, a reed is affixed above or below its slot rather than in the plane of the slot, it responds more easily to air flowing in the direction that initially would push it into the slot, i.e., as a closing reed.

This difference in response to air direction makes it possible to include both a blow reed and a draw reed in the same air chamber and to play them separately without relying on flaps of plastic or leather (valves, wind-savers) to block the nonplaying reed. An important technique in performance is bending: causing a drop in pitch by making embouchure adjustments. It is possible to bend isolated reeds, as on chromatic and other harmonica models with wind- savers, but also to both lower, and raise (overbend, overblow, overdraw) the pitch produced by pairs of reeds in the same chamber, as on a diatonic or other unvalved harmonica. Such two-reed pitch changes actually involve sound production by the normally silent reed, the opening reed (for instance, the blow reed while the player is drawing).