Double Overhand Knot

Add one extra wrap to a plain overhand knot before tucking the working end through, and the result is bulkier, rounder, and noticeably more resistant to being dragged back through a small opening - exactly the property needed for the stopper knot climbers and rope-access technicians tie at the tail end of a rope, so it can't feed all the way through a belay device or rappel rack even if a hand slips off the brake.

The extra wrap doesn't fundamentally change the knot's mechanics; it's still an overhand-family knot bent tightly back on itself, and it costs roughly the same proportion of the rope's strength a plain overhand does. What changes is the finished shape - two wraps rolling together into a rounder, more substantial lump than a single wrap produces, which is harder to pull through a gap by accident.

That's also why it's the standard core structure inside the double fisherman's knot and blood knot family, where two double overhand knots tied around each other's standing parts form a bend strong enough to join two climbing ropes. The double overhand isn't usually the final knot in those cases - it's a load-bearing building block that gets its real strength from being tied twice, interlocked, rather than standing alone.

As a standalone safety stopper, it's tied at the very end of a rope's tail specifically because a hand losing its grip on the brake strand during a rappel or lower needs the rope to physically stop feeding through the device rather than running out entirely - a documented and genuinely serious failure mode the stopper knot exists to prevent, which is why most guide services and climbing gyms treat a missing tail-end stopper as an automatic safety-check failure.

Tying it correctly matters more than it might seem: the two wraps have to roll together into a single compact coil when the knot is pulled tight, not cross over each other into a lumpy, uneven shape. A rushed or sloppy double overhand can end up smaller and less bulky than intended, quietly undermining the entire reason it was tied in the first place. Guides checking a client's rappel setup typically glance specifically at the tail knot's shape and size, not just its presence, for exactly this reason.

Outside climbing, the same knot shows up constantly in everyday cordage as a simple, reliable way to keep a rope end from fraying through a grommet or from pulling back through a drawstring channel, since its bulk works just as well against a fabric opening as it does against a metal belay device.