Tongue and groove is a method of fitting similar objects together, edge to edge, used mainly with wood, in flooring, parquetry, paneling, and similar constructions. Tongue and groove joints allow two flat pieces to be joined strongly together to make a single flat surface. Here's the tool for making your own tongue and groove! It's plug and play, attach to your router and it's ready to use!
A strong joint, the tongue and groove joint is widely used for re-entrant angles. In expensive cabinet work, glued dovetail and multiple tongue and groove are used.
A well-made tongue-and-groove joint presents a gapless seam along the line, or face, with a tongue that fits snugly into the groove.