A housing joint is a very simple, very strong woodworking joint that joins two workpieces at right angles by cutting a slot, or dado, into one piece and fitting the other piece into it. Commonly found in bookcases and other shelving units, as well as drawers and cabinet carcasses, they're favored by woodworkers because of the ease of their construction, typically requiring that only one workpiece be cut. A housing joint consists of a dado perpendicular to the front edge of a workpiece and no deeper than a third of its thickness. The width of the dado matches the width of a second workpiece, which fits into the dado and creates a strong right-angle joint.
One of the most common uses of a housing joint is in the construction of shelving units, but they're also frequently used in the construction of cabinet carcasses. When building shelves, identical dadoes are cut into each side of the unit, perpendicular to the front edge, to the exact dimensions of the end of the shelf. When both dadoes are properly cut, the shelf will fit perfectly in place. Modern housing joints are nearly always cut with a router and a jig, but were carefully cut by hand in the days before power tools.
If the cut is made all the way through from the front to the back of the supporting sides, the joint is called a "through" housing joint, and no cut is made to the shelf itself. A drawback of the through housing joint, though, is that the elements of the joint are visible from the front of the workpiece, which often detracts from its aesthetic appeal. An alternative is the "stopped" housing joint, in which the cut is stopped short of the front, and sometimes the back, of the side support. A stopped housing joint generally requires that the corners of the shelves be trimmed to facilitate fitting the ends into the stopped dadoes. When viewed from the front, the dado won't be visible, giving a clean, neat appearance.
Housing joints are one of a handful of strong, simple joints often used to make right-angle joints. It has three elements: two sides and a bottom. A "rabbet" joint is built almost the same, but is cut into the end of the workpiece so that it has only one side and a bottom. When a rabbet joint is used, the second workpiece is usually cut to fit. A butt joint is made by butting two pieces of wood together, edge to face, without any cutting at all. The two pieces are usually secured with nails and screws, as there would be too much pressure and stress for glue to secure a butt joint.
Like most woodworking joints, housing joints are usually secured with carpenter's glue and clamped until the glue has cured. Depending on the joint's use, nails or screws can also be driven through the bottom of the dado and into the edge of the shelf, but most woodworkers disdain the use of screws or nails in their joinery. Another, more popular way to strengthen housing joints is to secure them with glue blocks. These are simple blocks of wood that fit snugly into the angles of woodworking joints and are glued in place, providing additional support to both elements of the joint and reinforcing the joint itself.