Picking the right impression tray to use is an important step in taking your own impression. Each kit comes with an adult large and an adult medium tray for you to try out. Prior to mixing the putty, insert both trays into your mouth to see which of the two fit best. When evaluating the fit, keep these two requirements in mind:1. The tray needs to be long enough so that it covers all of your molars. If the tray does not cover your molars, we won't have a full impression of your teeth and can only extend the guard as far back as impressed. 2. Your teeth should not touch any of the side walls of the impression tray. There should be space between your teeth and the impression tray on both sides of your teeth. If you find yourself hitting any of the side walls with your teeth, it's best to use a larger tray.