A button accordion is a type of accordion on which the melody-side keyboard consists of a series of buttons. This differs from the piano accordion, which has piano-style keys. Erich von Hornbostel and Curt Sachs categorize it as a free reed aerophone in their classification of instruments, published in 1914. The sound from the instrument is produced by the vibration of air in reeds. Button accordions of various types are particularly common in European countries and countries where European people settled. The button accordion is often confused with the concertina; the button accordion's buttons are on the front of the instrument, where as the concertina's are on the sides and pushed in parallel with the bellows.