As the name implies, these O-Rings are used to seal Straight Threaded Tube Fittings. As with other O-Rings in static application designs, they make a seal between two mating components. Unlike most O-Rings, they are not completely housed in a rectangular groove or gland. The threaded male tubing fitting does contain a radiused partial groove that retains the O-Ring. The mating part, typically a valve or cylinder port, has a chamfered face that compresses the O-Ring diagonally. The straight threads of both components allow the tube fitting to set completely, causing the O-Ring to be nearly fully compressed. This technique is often referred to as an O-Ring Boss system.