The harmonium is one of the most common instruments used in Kirtan, Vaja Kirtan, Bhajan, and Qawwali Indian Music.

The harmonium is a reed instrument with a sound similar to the accordian. The Harmonium was a European reed instrument, which was brought to India by the British for preaching Christianity. The Harmonium was originally a foot-pump organ. To conform to the Eastern tradition of sitting on the floor while singing devotional music, the Harmonium was modified to a hand-held pump. The Harmonium keys are set similarly to those of the piano with only a 2 1/2 to 3 octave range.  The keys are usually thinner and not fully spring loaded. Some professional Harmoniums have a special feature called a ‘scale changer’, which allows one to transpose the entire scale of the Harmonium up seven pitches.

Special notes when purchasing and maintaining a Harmonium:
If buying a matching set of two Harmoniums, play them both to make sure that the middle “C” or the “Sa” pitch matches identically in both. The number of pumps or bellows will determine the amount of air used to produce sound. There should be around 3 bellows and the air should flow freely. There are two types of Harmoniums: standard and portable (folding).

Generally, the portable does not produce a rich sound, but is excellent for travelling. An exception is the ‘scale changer’ which is usually the portable type. The type of wood the Harmonium is composed of will determine the richness of the sound. Harmoniums made of teak will weigh more and have a distinct rich sound. The reeds used will also determine the quality of sound. Some create a high pitch sound, which many females prefer to use. When possible, buy a Harmonium, which has been used and played with for 4 – 6 months, it will be worked in and the true sound will be evident.

Keep the Harmonium covered and in a dry place when not in use. Moisture and dust will cause it to lose its tuning. When closing the pump, press the keys until all sound is diminished. This empties all the air, keeping the chamber void of moisture when not in use. Most Harmoniums today are made in Calcutta, Bombay and Punjab. When possible, go to the source or purchase an instrument from a trusted musician, to obtain an instrument of correct value and quality.

http://en.wikipedia.org/wiki/Harmonium