Crooked teeth are straightened by the use of special devices and techniques [orthodontics]. An orthodontist is a dentist who has been specially trained to use a selection of braces (orthodontic appliances), both fixed and removable, to improve the appearance and health of the teeth and gums.
Removable braces are very useful at moving either individual or groups of teeth, for example for correcting a deep bite or widening a narrow upper jaw. As such they can be used at an earlier age than the fixed brace. This type of treatment usually precedes fixed brace treatment. The big advantage of removable braces is that they are easily cleaned.