harmonica, either of two musical instruments, the friction-sounded glass harmonica or a mouth organ, a free-reed wind instrument whose invention is often attributed to Christian Friedrich Ludwig Buschmann (maker of the Mundäoline, Berlin, c. 1821). Whatever its origins, the contemporary harmonica consists of free metal reeds set in slots in a small metal-enclosed wooden frame and supplied with wind through two parallel rows of wind channels. The notes of the diatonic (seven-note) scale are obtained by alternately blowing and sucking, with the reeds positioned to sound by alternate directions of wind flow. A note can be played either by puckering the lips to single out the desired mouth hole in the instrument or by covering unwanted holes with the tongue (usually considered a more-advanced technique). One or more notes can be sounded at a time. In chromatic (12-note scale) models, a finger-operated stop selects either of two sets of reeds tuned a semitone apart. Compass ranges from two to four octaves, and harmonica bands include bass models. The virtuosity of some modern players stimulated several eminent composers (including Heitor Villa-Lobos and Ralph Vaughan Williams) to write for the instrument.
The harmonica is only one of several free-reed mouth organs. The oldest, the Chinese sheng, which was introduced to Europe in the 18th century, provided the principle both for the harmonica and for the short-lived symphonium, in which buttons channelled air from a single mouth hole to the proper reeds.