The Bass Appliance is a combined activator and headgear conceived by Neville Bass. It is a very effective appliance that can be used for the successful treatment of back-bite in the lower jaw even if it is only worn at night. Similar appliances are the Teuscher-Activator, the Hansa-Appliance and the Van Beek Activator.
At the end of the day, whichever Activator-Headgear combination is actually preferred by an orthodontist is unimportant since the crucial principles of action are the mandibular incision and the pull-up. This is the same for all appliances.