A dado housing (UK) or trench (Europe) is a slot or trench cut into the surface of a piece of machinable material, usually wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

. When viewed in cross-section, a dado has three sides. A dado is cut across, or perpendicular to, the grain and is thus differentiated from a groove

Groove (joinery)

In joinery, a groove is a slot or trench cut into a member which runs parallel to the grain. A groove is thus differentiated from a dado, which runs across the grain....

 which is cut with, or parallel to, the grain.

A through dado involves cuts which run between both edges of the surface, leaving both ends open. A stopped or blind dado ends before one or both of the cuts meets the edge of the surface.

Dados are often used to affix shelves to a bookcase

Bookcase

A bookcase, or bookshelf, is a piece of furniture, almost always with horizontal shelves, used to store books. A bookcase consists of a unit including two or more shelves which may not all be used to contain books or other printed materials. Shelves may be fixed or adjustable to different positions...

 carcase. Combined with a rabbet

Rabbet

A rabbet is a recess or groove cut into the edge of a piece of machineable material, usually wood. When viewed in cross-section, a rabbet is two-sided and open to the edge or end of the surface into which it is cut....

(rebate) on an adjoining piece, they are used to make the rabbet and dado joint, sometimes used in case goods.