Harmoniums have keys like the piano and a bellows that is pumped with one hand. The sound is created by the air streaming through tuned metal reeds. Depending on the type of harmonium, there are registers for lower and higher pitch, for doubling the octaves, regulation of the volume, vibrato and various bordunes. Indian harmoniums have a practical size and a soft voluminous sound. They were originally used as travel organs by Christian missionaries, and when they were brought to India in the 19th century, the locals were so charmed by them that they began making them and using them for their own temple functions. These days, they are made almost exclusively in India and Pakistan.