A D-ring is an item of hardware, usually a metal ring shaped like the letter D. It may be used at the end of a leather or fabric strap, or may be secured to a surface with a metal or fabric strap. For light loading applications such as clothing and luggage, D-rings made of plastics such as nylon may be used, as they weigh less and are impervious to rusting.
A common application for a D-ring is at the end of a tow-rope or chain, to allow one to create a bow around an item or part of an item that is being towed by a vehicle. They are commonly used with a chain to tether a boat to a dock or tree when it is being moored.
A D-ring when found on an M16 or variant type rifle is used to increase the pressure on the extractor and reduce malfunctions. The device is actually a rubber grommet shaped like a "D" and fits over the extractor spring adding tension to it.
D rings are commonly used in theatrical applications where a piece of scenery has to be lifted or "flown". D rings are attached to the tops or bottoms of "flats" with a "drift line" and turnbuckle attached to adjust the 'trim'.