The term ‘harmonium’ may refer to one of two instruments. The first is a kind of reed organ originating from the early 19th century. The sound is produced by blowing (or in some cases drawing) air over reeds by a set of bellows. Air is driven through the bellows by two pedals, operated by the left and right feet pushing the pedals down in alternate strokes, while the hands play a keyboard to produce that actual notes. The harmonium is usually used as substitute for the organ to accompany hymns, but has also been used in orchestral music such as the bagatelles of Antonín Dvořák and Ariadne auf Naxos by Richard Strauss.
The second kind of harmonium is a portable hand pumped instrument. It has an accordion style bellows section at the back, which is pumped usually by the left hand, while the right hand plays the small keyboard at the front, which spans about three and a half octaves. This type of harmonium is popular in India and is used in many kinds of indian music but especially in devotional songs.