The term reed organ normally refers to a keyboard instrument in which sound is produced by free reeds. Accordions and concertinas are examples of small, hand-held reed organs. Free reeds are thin, flexible strips of metal, usually brass, that are secured at one end over or under close-fitting openings in plates that are mounted over a wind-chest. Suction or wind pressure causes the free end of the reed to vibrate in and out of its aperture when its key is depressed to open a valve, or pallet, in the wind-chest. The pitch of each reed is determined by its length, though tuning can be accomplished by thinning or weighting the reed. No resonator is required to modify the tone quality, which is affected chiefly by the shape, thickness, resilience, and curvature of the reed.

It is not known when the free reed was first introduced to Europe from Asia, where as early as 1100 bc free reeds were incorporated into mouth organs such as the Chinese sheng. The sheng itself was known in Europe by 1777, by which time free reeds had already appeared in experimental organs. The tuning stability, small size, and low cost of free reeds were quickly recognized as virtues, and by the early 19th century small organs using free reeds in place of or in addition to pipes and with bellows pumped by the player’s feet were being manufactured in Europe and the United States. Occasionally free reed stops appeared as an adjunct to pianos and in mechanical instruments such as Johann Nepomuk Maelzel’s panharmonicon, first exhibited in Vienna in 1804.

Because of their simplicity, portability, and ease of maintenance, by the mid-19th century reed organs had become popular domestic instruments, rivaling the piano in numbers though not in quality or quantity of repertoire. In churches and other institutions, especially less affluent ones in rural areas, reed organs with up to three manuals, a pedal keyboard, and five or more sets of reeds often served in place of costlier pipe organs. The versatile and expressive harmonium (a compact reed organ patented in 1842 by the Frenchman Alexandre-François Debain and later developed by others) attracted the efforts of major composers such as Antonín Dvořák and Cesar Franck.

While European reed organs generally operated with wind under pressure (supplied by feeder-bellows), a type that employed suction to sound the reeds was developed in the United States, notably in New England, where seraphines, lap organs, and melodeons (as some varieties were called) were patented and manufactured in great numbers after about 1830. In 1847, Emmons Hamlin, an employee of the George A. Prince melodeon factory in Buffalo, N.Y., greatly improved the tonal quality of free reeds by bending them in various ways; the Boston firm that Hamlin founded with Henry Mason in 1854 became an international leader in producing “cabinet organs” of modest size.

Although reed organs continued to be manufactured into the 20th century and were occasionally employed in ensemble music by serious composers such as Arnold Schoenberg and Kurt Weill, they suffered from defects such as sluggish response, cloying tone, and limited expressive capability. In the face of unrelenting competition from cheap upright pianos (which they often resembled in appearance) and, later, from electronic instruments, reed organ production declined in the second quarter of the century, though fine old examples in playable condition remain numerous. Production of small reed organs has continued in India, where they provide drone accompaniments for, for example, sitar music.