Twin Block is made up of two components, an upper and lower, which work together to posture the lower jaw forward via a screw mechanism. This frees up the otherwise ‘lock-in’ lower jaw to grow to its full potential. The upper (and sometimes lower) also has an expansion screw to widen the upper arch. It can be used on people of all ages, but is more commonly used in children, who are in their growth stage, to treat an underdeveloped lower jaw. It is usually a first phase appliance followed by braces or a retainer.