A jack chain is a type of chain made of thin wire, with figure-eight-shaped links, and loops at right angles to each other. Jack chains are often used to suspend fixtures such as lights or signs, for decorative purposes, or as part of a cable lock.
Jack chain may be manufactured as either single-jack chain or as double-jack chain. If double-jack, the lower loop is formed of two strands of wire rather than just one as in a single-jack.
Before the days of lavatory cisterns being close to the pan, jack chains were often used to release the cistern plug.
Other meanings of "jack chain"
A jack chain is a tool attached to a toothed chain for moving logs.