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Most diatonic button accordions have a "single-action" (or bisonoric) keyboard, meaning that each button produces two notes: one when the bellows are pressed or pushed (closed) and another when the bellows are drawn or pulled (opened).
![excelsior accordion de botones excelsior accordion de botones](https://i.pinimg.com/736x/3c/50/29/3c50292cac37675b2898cc5389992d15.jpg)
![excelsior accordion de botones excelsior accordion de botones](https://www.jimlaabsmusicstore.com/wp-content/plugins/wp-easycart-data/products/pics1/b352_53e6691a9f511ae1c00113e2cde88c79.jpg)
(Historically, the term melodeon was applied to various 19th-century free-reed organs.)