The accordion is a portable box-shaped bellows-driven instrument. It belongs to the free-reed aerophone family, alongside the harmonica and the concertina. Sound is produced by air moving across metal reeds inside the casing. The player expands and contracts the bellows while pressing buttons or keys on either side. There are two main types: the 'Piano Accordion' (with a piano-style keyboard for the right hand) and the 'Button Accordion' (using buttons for both hands). The accordion is a staple in many folk traditions worldwide, from French Musette and German Polka to Argentinian Tango (via its cousin, the bandoneón) and American Zydeco. Its ability to play melody, harmony, and bass simultaneously makes it a 'one-man orchestra.' In the early 20th century, it was one of the most popular instruments in the world before being eclipsed by the guitar in pop music.