Jack Arch 
This arch, also called a flat arch, straight arch, minor arch, segmented arch, or multi-centered arch, has a horizontal top surface, with a span of up to 6 feet, can support loads up to 1,000 pounds per square foot, with a rise to span ratio of 0.15 or less. 

Jack Hammer 
Device with a chisel or pointed bit, which is either pneumatic or electric and is used to break or cup up a surface. 

Jack Plane 
Hand tool, which consists of a long base plate with a sharply angled cutting blade, extending through the slot in the base. When the base plate is slid along a piece of wood, the blade shaves off thin layers of the wood, unless the extension is adjusted for thicker layers. 

Jack Post 
Screw jack or lifting device with wide based footing and a top plate for spreading the load. Used for raising and supporting a horizontal member, which is sagging, possibly a cracked floor joist, it can be left in place permanently. 

Jack Rafter 
Roofing rafter reaching from a valley to a ridge or from a hip to a wall top plate. 

Jackscrew 
Type of lifting device, which adjusts its height and the height of its load, by turning a threaded shaft in an internally-threaded sleeve. 

Jackshaft 
Also called a countershaft, this solid, short round shaft is used for the transmission of power from a motor to a working part. 

Jack Stud 
Short stud, which does not reach from the floor to the ceiling, but only from the floor to a window, for example.

Jahn Forming System 
Trade name of concrete forming accessories that are used to hold sheets of plywood in place when making concrete forms. The accessories include brackets, clips and ties. 

Jalousie Window 
Type of window made up of a number of rectangular panes of glass or louvers, placed in slots in a frame, one above the other. The panes are held in place by a pinned connection that rotates, allowing the panes to be opened simultaneously to provide a flow of air. When closed, the panes overlap slightly, preventing rain from entering the structure, however they do not seal tightly so this type of window works better in mild climates. It is often referred to as a louvered window. 

Jam Nut 
A nut, which is tightened against the primary nut, to keep it from loosening, when the primary nut is tightened to the needed torque. A jam nut may be the same as the primary nut or may be made of stamped sheet metal. 

Jamb 
A piece of a framed opening, such as the bottom, side or top of a door or window opening. 

Jamb Block
Block that is formed with a special slot to hold the jamb of window or doorframes. 

Jamb, Head 
The top window frame or doorframe member. 

Japan Wax 
This wax is used as an ingredient in furniture polish and floor waxes and comes from the fruit of the oriental sumac plant. 

Japanese Wood Oil 
Also called Tung oil or China wood oil, this drying oil is taken from the seed kernels of the Tung tree and is used in making wood finishes. 

Jarno Taper 
The standard taper, which is 0.600 inch taper per foot (tpf), for lathe headstock and tailstock sockets. The taper is used to grip similarly tapered objects. 

J Bend 
The trap section, with a 180-degree bend, of a multi-piece P-trap, which is also called a return bend. 

J Factor 
Factor used by appraisers and investors to determine the changes needed in operating income to obtain a desired rate of return. This factor is determined after consideration of the location, rental, similar properties and cost of maintenance. 

J Hook 
Electrical wire in which a hook like bend in its end helps to shape and fit it around an electrical terminal. 

Jeopardy 
Danger, hazard, risk, or peril. 

Jetting 
1. Digging holes by directing water under high pressure to a certain spot. 
2. Injection of water into the ground with a metal pipe so that the action of the water will consolidate the loose earth to help settle unstable ground or backfill. 

Jetty 
Projecting structure or part of a building. 

Jib 
Projecting from the main body of a crane, this arm can be swiveled around to reach and move loads. 

Jigsaw
Power saw that cuts curved, irregular and intricate patterns with its thin blade that moves up and down 

Jimmy 
A short crowbar. 

Jitterbug 
Hand tamper, also referred to as a concrete tamper, which consists of a metal grill with long handles. It forces large aggregate beneath the wet concrete surface to leave it smooth and finished.