Tongue and groove is a time-tested form of woodworking joinery, often used in hardwood flooring or paneling. Before the advent of plywood sheeting, tongue and groove techniques were used to join smaller strips of wood to form a larger flat panel without the need for nails. Modern carpenters rarely use the tongue and groove technique, since other methods of joinery are not as time-consuming.

In tongue and groove joinery, a wooden slat receives two different treatments. One pass with a special bit creates a lengthwise protuberance or tongue which extends out from the slat. The opposite side of the wooden slat receives a channel or groove that will receive the tongue perfectly. The tongue side of each wooden slat or panel slides into the groove side of the next, and so on. Some pieces may only contain a tongue or a groove and serve as finished borders.

Tongue and groove joinery works best for hardwood flooring because smaller individual pieces can be installed to form a very large finished product. Imagine trying to bring in an entire floor and dropping it in the middle of the room. Since each slat or panel in tongue and groove joinery is engineered to fit another, pieces don't have to be customized. Installers can simply start at one end of a room and begin fitting tongue and groove slats together. If the standard length of a slat becomes problematic, it can be cut to size and moved into position.

Tongue and groove techniques are also used to form wall paneling and wainscoting. This style of joinery is often seen in older homes built before the development of large paneling sheets. Individual wooden slats are slid into place along the walls and the corners are covered with decorative moldings to hid the edges.

Tongue and groove hardwood flooring is not usually designed to be nailed or glued into place. The floor must be allowed to expand and contract naturally as the humidity level changes. The individual panels are generally held in place by the compression of the installation process. Essentially, the outer pieces put force on the center pieces to create stability. The pieces are free to slip back and forth, but the tongue and groove connection should keep them from moving up and down.