The mouth piece and carry case used for teeth whitening it comes with 2 mouth pieces, one each for upper and lower teeth. 

The mouth piece is generic in size, placed in hot water and let to soak for a few minutes it will soften slightly so that you can then place it over your own teeth, (one for the upper set and the other for the lower) allow to cool and set. The plyable mouth piece will mould itself to fit over your teeth and once set this will be used whenever you are self administering the home whitening system.