Terms of the Trade: What Is a Tongue and Groove Joint?

Several pieces of lumber with tongue and groove joints.

The tongue and groove joint allows for two flat pieces of wood to be joined together to create a new flat surface. For the joint to work, each piece of wood needs to have a slot (the groove) along one edge and a thin ridge (the tongue) on the opposite edge. Usually this allows for a tight fit and glue is not needed.