The document provides information about fixed functional appliances. It begins with an introduction and definition of fixed functional appliances. It then discusses the history, indications, and modes of action. The document classifies fixed functional appliances based on mode of action, type of force produced, and other categorizations. It specifically describes rigid fixed functional appliances like the Herbst appliance. It discusses the original Herbst design and modifications. The document outlines the treatment results and problems associated with the Herbst appliance. It also describes other fixed functional appliances like the acrylic splint Herbst.