Your orthodontist may suggest the use of an appliance at some point in your orthodontic treatment. Here are great photos from Gergen’s Orthodontic Lab in Phoenix, AZ. We non-experts call some of them just retainers, but there are many different styles and types of appliances that serve different specific purposes. Some connect to cemented bands and some can be removed for eating and brushing.
For children, a common appliance is an expander that widens the upper arch to deal with crowded teeth, usually in children. For an open-bite problem, where the front top and bottom teeth don’t touch, another appliance type is called a tongue crib; again usually used for children/preteens, this design can help train the tongue to stay back instead of constantly pushing the teeth apart. Sometimes, both an expander and a tongue-crib are combined.
A splint appliance protects the teeth from grinding problems while someone is sleeping; I’ve actually ground through mine, so I know they work! And one typical removable retainer is called a Hawley, made from both plastic and some wires that help the retainer stay in place and do its job.
Whatever the design, wear your appliance for the hours, days, years, or lifetime as stated by your orthodontist. It can’t do the job if you don’t wear it!