Tongue and groove joint
Also called: tongue and groove, tongued and grooved, t and g, loose tongue, cross tongue
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A tongue and groove joint joins two boards edge to edge by cutting a thin ridge, called the tongue, along the edge of one board, and a matching slot, the groove, along the edge of the other, so the ridge locks into the channel and the two faces come out flush and cannot twist apart. Audels (1923) described it plainly: the tongue is formed "on the edge of one of the pieces to be joined, the groove being in the other".
Flooring and boarding
Fairham (1920) called it a joint "used in one form or another throughout the whole of the woodworking trades", covering everything "from the laying of flooring boards to the construction of light cabinet work". Its oldest and commonest use is in floors and matchboarding, the narrow tongued and grooved boards used to line walls and ceilings: Hasluck (1897) judged it "not a very strong joint, but it is cheaply made". In flooring it earns its keep by hiding the fixings. Fairham (1920) described ballroom and skating rink floors, where the tongue and groove is worked so that it covers the nails. Hidden, or secret, nailing works this way generally: each board is fixed down through the base of its groove, and the tongue of the next board is driven home over that fixing, so no nail heads show on the walking surface. Modern flooring trades still shorten the description to the initials PTG, for planed, tongued and grooved, and treat it as the ordinary way to lay a timber floor, in place of the plain square-edged boarding found in older buildings. The joint also does practical work against draughts. Audels (1923) explained that the leakage which comes through the cracks of a plain, butt-jointed floor, where the edges simply touch without interlocking, is overcome by fitting a tongue and groove joint instead, and Hasluck (1907) described solid blocks used to fill between floor joists, tongued and grooved together with their grain set on end, specifically to stop air passing between them.
Loose tongues and corners
Where a board's own edge cannot supply a tongue, joiners fit a separate strip instead. A loose tongue, a thin strip of wood glued into matching grooves cut in both pieces, lets the same locking principle work on a mitre, where two pieces meet at an angle rather than running flat edge to edge. Hayward (1950) recommended it as a simpler alternative to a mitre dovetail for strengthening a mitred corner. Ellis (1908) described a related use in curved joinery: linings jointed at the springing, the point where a curve begins, with a stub tenon or a grooved and cross tongue, and a sash stile and head joined on the radial with a cross tongue set into the rebate side. A related form serves box and case corners in furniture making. Frid (1979) described a corner tongue and groove for solid wood or plywood, and for a plywood corner specified that the grain of the corner piece has to follow the same direction as the edging it sits against.
Wood movement
Solid wood panels held in a plain tongue and groove will still move with changes in humidity, since wood expands and contracts across the grain far more than along it. Hoadley (1980) noted that a wide bevel worked around a panel's edge hides this seasonal movement from the eye, movement that would otherwise show plainly if the same panel were fitted tightly into a simple tongue and groove.