Double fisherman's knot

The double fisherman's knot or grapevine knot is a bend. This knot and the triple fisherman's knot are the variations used most often in climbing, arboriculture, and search and rescue. The knot is formed by tying a double overhand knot, in its strangle knot form, with each end around the opposite line's standing part.

Usage
A primary use of this knot is to form high strength (round) slings of cord for connecting pieces of a climber's protection system.

Other uses
This knot, along with the basic fisherman's knot can be used to join the ends of a necklace cord. The two strangle knots are left separated, and in this way the length of the necklace can be adjusted without breaking or untying the strand.

Security
Dyneema/Spectra's very high lubricity leads to poor knot-holding ability and has led to the recommendation to use the triple fisherman's knot rather than the traditional double fisherman's knot in 6 mm Dyneema core cord to avoid a particular failure mechanism of the double fisherman's, where first the sheath fails at the knot, then the core slips through.