Abstract: A dental device is described in the form of a mandibular splint having two arch members interconnected together by a fastener. The fastener of the present invention includes a flange for locating the fastener in the correct location, two tabs arranged to extend in opposite directions from opposite sides of the body of the fastener for connection to two substantially arcuate wings which are pivotally connected to the fastener. The wings are adjustable in orientation both with respect to each other and with respect to the fastener and are adjustable in length. The fastener is also provided with an adjustable hook or similar arrangement for interconnecting the two arches together in use to maintain the jaws of a person suffering from a sleep disorder at a predetermined spaced apart relationship to open the breathing passages to ameliorate the sleep disorder. Opening of the breathing passages reduces the chances of a person snoring as well as treating other sleep disorders.