The back refers to the lower part of a string instrument. Usually it is made of maple wood. Other hard woods such a poplar or willow have been and still are being used. Maple wood seems to produce the best sound, however. The back can be fashioned out of two separate pieces that are glued together in the middle. In instruments of the violin family (violin, viola and cello) the back and the belly are curved. The curvature is carved out of solid wood and the perfect sound is achieved using various thicknesses of wood.