The bandoneón once dominated the tango ensemble; its sound and style still define the genre today. Transported to Argentina by German immigrants, this instrument visually resembles the accordion. Its 38 right-hand buttons play in the up-per and middle ranges, and its 33 left-hand buttons produce the low register. Originally designed as a diatonic folk accordion that played in only a few keys, the bandoneón evolved to include more buttons, which allowed it to substitute for the organ in small German churches. Normally on keyboard instruments, neighboring tones occupy adjacent keys, but because of the haphazard way that the bandoneón evolved, its complex physical layout defied logical explanation. Fiendishly difficult to play, it has attracted a group of performers who have built the core repertoire of the tango around this impractical (but fascinating) instrument.