Piano-organs are musical instruments that combine the features of a piano and an organ. They are also known as harmoniums, reed organs, or pump organs. Harmoniums were invented in the 19th century and were very popular in the Victorian era. They have a keyboard similar to a piano, but instead of hammers hitting strings, they use air and reed pipes to produce sound. Reed organs were popular in churches and homes, and they have been used in some classical music compositions. Pump organs use foot pedals to create air pressure to produce sound. The terms piano-organs, harmoniums, reed organs, and pump organs are often used interchangeably and refer to the same type of musical instrument.