ARMONIO. — It is a keyboard instrument whose sound is produced by free reeds without resonators, placed horizontally beneath the keys. Air is supplied by a bellows activated by two pedals. It derives from the ancient "regal," a small portable organ with reed stops without resonators or with very small resonators. After various examples built by C. Perrault, S. Erald, C. Buffet, Cavaillé-Coll, the Belgian (1809–77) constructed the first models of the present-day harmonium. Subsequent manufacturers introduced improvements: Vittorio Mustel (1815–90) may be considered the father of the modern instrument.
It can be constructed with two different systems: pressure or suction, depending on the different way in which air acts on the reeds. The complete set of reeds producing a sound of a specific timbre throughout the entire range of the keyboard is called a "game." The flute, clarinet, bourdon, oboe, English horn, bassoon, dulcian, aeoline, voix céleste, etc., are among the principal games, which, with respect to their pitch, can be of 16, 8, 4, or 2 feet. The mechanical means by which the performer uses the game and other components is called a "stop." The main mechanical stops are: expression, a stop of great aesthetic importance that gives the harmonium a distinctive character; percussion, very useful for the clear execution of agile passages and pianissimos; fixed forte, which opens or closes a kind of shutter; expressive forte (Mustel), for which the shutter is opened more or less by a small bellows depending on the varying pressure exerted on the pedals; deep sustain, by which one or more notes in the bass register can be held indefinitely without hindering the free movement of the two hands; super and sub octave; tremolo; mute. Two knee levers serve to operate a grille divided into two parts, one for the bass register and the other for the treble register, which opens or closes the sound. The two parts of the grille can be operated independently (double expression). Another device allows all the games to be opened simultaneously.
The most common type of harmonium is with a single keyboard, but there are also double-keyboard models with pedalboard. It is most commonly used to accompany choirs in churches and schools; however, the artistic harmonium tends to become a concert instrument, having been enriched by an important repertoire, particularly with works by C. Franck, L. Boëllmann, A. Guilmant, C. M. Widor, Max Reger, and S. Karg-Elert.