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.

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.

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 in furnitures and face nailing on floors.

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 boards. 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.

One of the following woodworking tools may be used to produce the tongue and groove:

• A four- or six-head moulder (for large quantities)
• A wood shaper (spindle moulder)
• A circular saw bench
• Suitable hand planes: a plough plane for the groove and a tongue plane for the tongue, or a combination plane
• A spindle router

Don’t mistake it with tongue-in-groove !

Source Wikipedia.