An orthosis includes an upper tray and a lower tray, which are designed to line the teeth of an upper jaw and the teeth of a lower jaw, respectively. A connection joins the trays. The upper tray has at least a first fixed base, and the lower tray has at least a second fixed base. The connection has a fixed length and is designed to be fixed, on the one hand, to the first base and, on the other hand, to the second base, in a position that has an advancing effect and that is adjustable by way of a control element integrated in at least one of the two bases.
