The bandoneon is a type of button-box accordion commonly used in tango music, originating in the "people of the seaport" area of Argentina and Uruguay. It is known for its 71 button format and is considered to be the heart and soul of tango music. It is a type of concertina and is greatly preferred by tango players, specifically the bisonoric bandoneon called 'Rheinische Lage'. It falls under the subgroup of "aero-phones" along with the accordion and concertina. Unlike an accordion, the bandoneon's manuals naturally fall open when a note is played, causing a change in its sound.