Jack Rafter: short rafter framing between a hip rafter and the wall plate; also a hip or valley rafter and ridge board.
Jalousie: a window which consists of parallel glass, acrylic, or wooden louvers set in a frame. The louvers are locked together onto a track, so that they may be tilted open and shut in unison, like a Venetian blind, to control airflow through the window. They are usually controlled by a crank mechanism.
Jamb: the top and two sides in a window or door frame that contact the sash or door. The word “jamb” comes from the French “jambe”, meaning “leg”.
Janka Hardness: measurement used in the janka hardness test for wood. It involves measuring the force required to embed a 11.28 millimeter (0.444 inch) steel ball into wood to half its diameter. This method was chosen so that the result would leave an indention 100 square millimeters in size. It is one of the best measures of the ability of a wood species to withstand denting and wear. It is also a good indicator of how hard a species is to saw or nail.
Jig: a mechanical device which acts as a guide to position material for accuracy of cutting or assembly.
Joiner: a crafsman who cuts and fits joints in wood that do not use nails, usually in a workshop environment since the formation of the various joints generally requires non-portable machinery.
Joinery: term used to refer to various types of joints in a structure.
Jointer: a woodworking machine used to produce a flat surface along a boards length.
Joist: one of the horizontal supporting members that run from wall to wall, wall to beam, or beam to beam to support a ceiling, roof, or floor. It may be made of wood, steel, or concrete. Typically, a beam is bigger than, and is thus distinguished from, a joist. Joists are often supported by beams and are usually repetitive.