What is "tongue and groove"

n. A joint, in flooring or panelling, made by fitting a ridge (''tongue'') along the edge of one board into a corresponding slot (''groove'') on another.

A strong joint, the tongue and groove joint is widely used for re-entrant angles. The effect of wood shrinkage is concealed when the joint is beaded or otherwise moulded. In expensive cabinet work, glued dovetail and multiple tongue and groove are used.

Tongue and groove is a method of fitting similar objects together, edge to edge, used mainly with wood, in flooring, parquetry, panelling, and similar constructions. Tongue and groove joints allow two flat pieces to be joined strongly together to make a single flat surface. Before plywood became common, tongue and groove boards were also used for sheathing buildings and to construct concrete formwork.

Each piece has a slot (the groove) cut all along one edge, and a thin, deep ridge (the tongue) on the opposite edge. The tongue projects a little less than the depth of the groove. Two or more pieces thus fit together closely. The joint is not normally glued, as shrinkage would then pull the tongue off.

In another assembly method, the pieces are end-matched. This method eliminates the need for mitre joints, face nailing, and the use of joints on or centres of conventional framing.

For many uses, tongue and groove boards have been rendered obsolete by the introduction of plywood and later composite wood boards, but the method is still used in higher-quality. Plywood may also be tongued all round to fit it flush into a framed structure, and plywood for sub-floors used in platform framing is often supplied with tongue and groove edges.

When joining thicker materials, several tongue and groove joints may be used one above the other.