These type of orthodontic appliances are one of the most important orthodontic tools especially for children and sometime for adults. Functional or orthopaedic appliance could be removable or fixed that works on both top and bottom teeth at the same time. Functional appliances are prescribed to those with increased overjet i.e. big gap between top and bottom teeth. It is commonly used in the UK and very effective appliance for children with big overjet. It is usually used full-time for 10-12 months followed by fixed braces to fine tone the position of the teeth.