A through dado (left) and a stopped dado
A dado (US and Canada), housing (UK) or trench (Europe) is a slot or trench cut into the surface of a piece of machinable material, usually wood. 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 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 carcase. Combined with a rabbet (rebate) on an adjoining piece, they are used to make the rabbet and dado joint, sometimes used in case goods.