accordion, musical instrument consisting of a rectangular bellows expanded and contracted between the hands. Buttons or keys operated by the player open valves, allowing air to enter or to escape. The air sets in motion free reeds, frequently made of metal. The length, density, shape, and elasticity of the reeds determine the pitch. The first accordions were made in 1822 by Friedrich Buschmann in Berlin. Bouton added a keyboard 30 years later in Paris, thus producing a piano accordion. The accordion is frequently used in folk music. See concertina.