What is the difference between a custom mouthguard and a boil-and-bite mouthguard?

A custom mouthguard is fabricated over an accurate model of the individual’s teeth, allowing the technician or clinician to control fit, coverage, material distribution and occlusion. A boil-and-bite mouthguard is a generic appliance softened and adapted by the wearer. Custom fabrication generally allows better control of retention and protective thickness.