Twin block appliances are a type of removeable orthodontic appliance, designed to encourage growth of the lower jaw. They are useful in growing patients where the lower jaw (mandible) has not developed as fully compared to the upper jaw (maxilla). This discrepancy leads to a large gap forming between the upper and lower front teeth.

The appliance comes in 2 parts (an upper portion and lower portion). When both parts are placed in the mouth, the lower jaw will be brought forwards at rest. Over time the lower jaw grows and adapts to this new position, improving the facial profile and reducing the space between the upper and lower front teeth.