Tongue and groove

Tongue and groove is a method of fitting objects together edge to edge. It is used for 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 wood-covered buildings and to construct concrete formwork (the wooden frames for concrete to be poured into).