The tongue and groove joint allows two flat pieces to be joined strongly together to make a single flat surface. When wood shrinks in changing temperature and weather the effect of wood shrinkage is concealed because the joint overlaps.
Tongue and groove fits together, edge to edge, and is used mainly with wood, in trailer decking, truck decks, truck interiors and floors and in many other non trailer applications.
Each piece has a slot (the groove) that is cut along one edge, and then a thin, deep ridge (the tongue) on the opposite edge. The tongue projects a little less than the depth of the groove. This is how two or more pieces thus fit together snugly. The joint is not typically glued, as shrinkage from temperature or weather changes would pull the tongue off or separate the wood.