The harmonium is a keyboard instrument in which the sound is produced by means of free reeds which are set in vibration by air. The accordion, for example, has a similar tone generation. By alternately stepping down two pedals, bellow is driven with the feet while playing.
The harmonium was very popular around 1900 as a home organ, as a replacement for (smaller) church organs, but also as a concert instrument. In 1945-1955 there was a temporary second renaissance as an alternative to the many organs destroyed in the Second World War.
Besides original literature, numerous works with harmonium as orchestral instrument can be found, e.g. in the Petite Messe solennelle by Rossini for voices, two pianos and harmonium.
Since the spread of the electronic organ and the keyboard, the harmonium has fallen into oblivion for new music.