A concertina is a free-reed musical instrument with expanding and contracting bellows and buttons on both ends. It was independently developed in England by Sir Charles Wheatstone in 1829 and in Germany by Carl Friedrich Uhlig in 1834. Concertinas are used in various music genres, including classical, traditional Irish, English, and South African music, as well as tango and polka. They come in different systems, such as English, Anglo-German, and German types, with variations in keyboard layout and the production of unisonoric or bisonoric notes.