Tongue and groove joint
A tongue and groove joint joins boards edge to edge: a tongue milled along one edge fits a groove milled along the edge of the next. The fit keeps the faces level and the boards in line, which is why it is used for flooring, wainscot, panelling and wide glued-up panels.
Groove, not dado
Woodcraft’s definitions help here: a dado runs across the grain, while a groove runs with it. A tongue and groove is a groove along the edge, with the grain, and a matching tongue, so the two boards sit side by side rather than crossing.
Gluing or floating
In a glued panel the tongue and groove mainly lines the boards up during glue-up. In flooring and panelling the boards are usually left unglued so each can move with the seasons, and the tongue lets them shift a little without opening a gap through the joint. A small chamfer or bead on the front edges hides the line as the boards move.
How it’s cut
A matched pair of router bits, or a single reversible bit, cuts the tongue and the groove in a router table so they fit without adjustment. On the table saw the groove takes one or two passes with a dado stack or a rip blade, and the tongue is formed by cutting a rabbet on each face. Run every board with the same face against the fence so the faces line up.
Related joints
Sources
- Woodcraft, Craig Bentzley, "Joinery Made Simple: Rabbets, Dadoes, and Grooves", read 2026-11-07
Proportions marked as a traditional rule are shop tradition, not measurements. This site gives no joint-strength numbers: the published tests each used a different rig. How we source these numbers.