Various German concertina systems share common construction features and core button layout. In the United States, particularly in theMidwest where there are many German and Central European descendants, the term concertina often refers to the Chemnitzer best concertina. Chemnitzer concertinas are bisonoric and are closely related to the bandoneón, but with a somewhat different keyboard layout and decorative style, with some mechanical innovations pioneered by German-American instrument builder and inventor Otto Schlicht.A related variant is the Carlsfelder concertina of C. F. Zimmerman, unveiled in 1849,.and at the 1851 London Industrial Exposition.