Abstract: The invention is based on the realization that the pain associated with many conditions such as headaches (in particular, migraines and other severe repetitive chronic headaches) is caused by a muscle imbalance that can be traced to the masticatory muscles controlling the lower jaw. It has been found that interdigitation of the molar teeth of the upper and lower jaws is the principal factor that causes the muscle imbalance. The treatment, therefore, requires the unlocking of the interdigitation. In this regard a particular construction of splint to overlie at least the molar teeth of the lower jaw is disclosed.