Tongue and groove

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Solid parquet boards with tongues on the right sides of the boards and grooves on the left sides. Grooves are also visible on the near ends; the far ends are tongued.

Tongue and groove or T&G is a method of fitting similar objects together, edge to edge, used mainly with wood: 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 groove is deep. Two or more pieces thus fit together closely. The joint is not normally glued, as shrinkage would then pull the tongue off.

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 good-quality flooring. 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.

[edit] Methods

The tongue and groove may be cut in a number of ways, including:

A four- or six-head planer (for large quantities)
A spindle moulder (wood shaper)
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

[edit] Tongue-in-groove

Tongue-in-groove is similar to tongue and groove, but instead of the tongue forming part of one of the edges, it is loose, fitting between two identical grooved edges. The tongue material may be the same as the pieces either side, or not. For example, plywood flooring is commonly grooved at the edges, and plastic tongues are used to form the joint.