Tongue and Groove Joint

A tongue and groove joint cuts a protruding ridge (the tongue) along one board's edge and a matching channel (the groove) along the next board's edge, so the two interlock along their full length when slid or pressed together. It's the standard joint behind hardwood flooring, tongue-and-groove paneling, and a lot of panel-edge work where boards need to register against each other automatically during assembly.

Why the self-alignment matters

Unlike a plain edge-glued butt joint, a tongue and groove physically can't go together misaligned — the tongue only fits one way into the groove, which keeps adjoining boards flush on both faces without needing extra clamps or cauls to hold the alignment while glue cures. That self-registering behavior is the entire reason the joint exists; the strength gain over a plain edge glue-up is real but secondary to the alignment benefit.

Cutting the joint

A dado stack or a matched pair of router bits (a tongue-cutting bit and a matching groove-cutting bit, sold as a set) are the two standard methods. A table saw can cut both the groove and, with repeated passes, the tongue, though a router table with matched bits gets a cleaner, more consistent result across many boards — which matters a lot when milling enough flooring or paneling stock for a whole room.

Fit and wood movement

The tongue should fit into the groove with enough clearance to slide in without forcing, but not so loose that the boards can rock relative to each other. On wide panel or flooring runs, this clearance also has to account for real seasonal wood movement (see the Wood Movement Estimator) — a tongue and groove installed bone-tight in a dry winter can bind or even split boards once summer humidity swells the wood, which is why flooring installers deliberately leave expansion room at the room's perimeter rather than fitting every board drum-tight.

Groove vs. dado, one more time

A groove runs with the grain, along a board's length; a dado runs across the grain. The tongue-and-groove joint specifically uses a groove (with-the-grain) on one edge and a matching with-the-grain tongue on the other — worth keeping straight since "groove" and "dado" describe the same cross-sectional cut in two different grain orientations, not two different shapes.