A twin block is an orthodontic appliance developed by William J. Clark whose purpose is to lengthen the lower jaw by posturing it forward.
[1] If the jaw is growing during treatment, it naturally grows at a faster rate; otherwise the appliance is useless.
Treatment time varies, but is generally around 9 months.
Twin Blocks can also be used to cure an overbite, which can mean treatment may be slightly longer.
If this is the case, treatment can vary from 6 to 11 months.