The tongue and groove joint is a strong joint that's widely used as a method of fitting similar objects together, edge to edge, used mainly with wood, in flooring, panelling, and similar constructions.
Tongue and groove joints allow two flat pieces to be joined together in a strong position to make a single flat surface. Generally all playhouses are made with tongue and groove boards to add to the stability of the construction. Tongue and groove joints also allow the wood to move naturally without visible gaps appearing between the boards.
Each piece has a slot or groove cut all along one edge, and a thin, deep lip (the tongue) on the opposite edge. The tongue projects a little less than the depth of the groove, so two or more pieces fit together closely. These joints are not normally glued together as the movement and shrinkage in the wood would then pull the tongue off.
The snug fit between a tongue and groove joint will make sure your playhouse stays strong and secure for many years to come.