The trim covers the lap joints and locks the boards in place to prevent separation on uneven floor. The trim pack includes pre-drilled 3-one/four inch broad major rails and corner moldings that attach into the raised mattress with incorporated screws. View our online video below! Raised beds assemble in minutes. https://raised-wood-garden-beds86431.national-wiki.com/701002/cedar_for_raised_garden_beds_an_overview