The accordion is a portable box-shaped musical instrument of the hand-held bellows-driven free-reed aerophone family, sometimes referred to as a squeezebox. A person who plays the accordion is called an accordionist. It is played by compressing or expanding its bellows, while pressing buttons or keys, causing valves called pallets to open which allow air to flow across strips of brass or steel called reeds that vibrate to produce sound inside the body.

The instrument is sometimes considered a "one-man-band", as it needs no accompanying instrument; the performer normally plays the melody on buttons or keys on the right-hand manual, and the accompaniment—consisting of bass and pre-set chord buttons—on the left-hand manual.

It is often used in folk music in Europe, North America, Russia, and South America. It is commonly associated with busking. Some rock bands make use of the instrument, too. Additionally, the accordion is sometimes used in both solo and orchestra performances of classical music.

The oldest name for this group of instruments is actually "harmonika". It comes as a mixture of "aer", "monos" and "cassa", the first two words being Greek and the last Italian. They mean "air", "unit" and "box", describing the instrument.... Today, native versions of the name "accordion" are more often used, which is a reference to the type of accordion patented by Cyrill Demian, which concerned "automatically coupled chords on the bass side".

Construction

Accordions are made in a large number of different configurations and types; there is not yet one standard accordion. As such, what may be technically possible to do with one accordion could be impossible with another:

• Some accordions are bisonoric, meaning they produce different pitches depending on the direction of bellows movement.
• Others are unisonoric and produce the same pitch regardless of the direction of bellows movement.
• Some accordions use a chromatic buttonboard for the right-hand manual.
• Others use a diatonic buttonboard for the right-hand manual.
• Yet others simply use a piano-style musical keyboard for the right-hand manual.
• Some accordions are capable of playing in different registers than others.
• Additionally, different accordion craftsmen and technicians may tune the same registers in a slightly different manner, essentially 'personalizing' the end result, such as an organ technician might voice a particular instrument.

As such, the boundaries of what defines an accordion are perceivably broad.