The term jaw expansion refers to "widening" of the back teeth. In general, the upper set of teeth should fit around (outside) the lower set of teeth. As such, the upper jaw should be "wider" than the lower jaw. The upper jaw is usually the one undergoing expansion. Expanders can be fixed or removable. Fixed expanders are generally preferred over removable appliances, because removable appliances require patient cooperation to be successful. This book covers each aspect of expansion in brief.