A button accordion is a type of accordion on which the melody-side keyboard consists of a series of buttons rather than piano-style keys of a piano accordion. The first button accordion is credited to Franz Walther in 1850. Button accordions of various types are used especially in European countries and overseas countries where European people settled.
The diatonic button accordion generally has two or three rows of buttons, each row tuned to a certain key. (The Cajun accordion or single-row button accordion is also a diatonic accordion.) They are usually seen in ethnic music genres such as Irish, British, Cajun, and Norteño music. Most diatonic accordions are bisonoric, meaning that a button produces two different notes by pushing or pulling the bellows. Accidentals are either not included or provided on additional "helper" buttons.
The chromatic button accordion is unisonoric, meaning that each button produces one note, whether pushing or pulling the bellows. This accordion also has 3-5 rows of buttons, but unlike the diatonic button accordion, it can be freely played in any key, usually with identical fingering patterns. This type of accordion is very popular in Parisian 'musette' music. It is used widely in Europe countries, including for ethnic dance music, often alongside diatonic or piano accordions, not in place of. However, chromatic accordions, like piano accordions, are generally more suitable to 'serious' music such as Classical and Jazz than diatonic instruments are (not that there aren't any virtuoso diatonic players or any serious music composed for them). In Europe, Chromatics are more common than piano accordions, while in North America it is the other way around.
All accordions and concertinas have three main components: the reeds, bellows, and buttons or keys. The accordion has reeds on both the treble and bass sides. The press of a button or key opens a valve to allow air to pass through a reed/reeds to make a sound when the bellows are pumped in or out. Some accordions have switches which select different reeds banks to give different tonal qualities, much as a pipe organ does. Most diatonic instruments lack switches, though there are some made by companies such as Hohner, as well as the one-row 'Cajun'-type boxes which have usually 3 or 4 stops on top of the box as switches (making it even more akin to a pipe organ). But it is generally more common to find switches on a chromatic or piano accordion.