Autoblock Knot
Also known as: French Prusik
hitch knot
Life-safety notice
TyingPoint's illustrations are for learning. For any life-safety use — climbing, rope access, rescue, or load-bearing rigging — seek qualified in-person instruction, and always inspect and test every knot before you trust your weight (or anyone else's) to it.
Rappelling always carries one specific risk that no amount of practice fully eliminates: a hand coming off the brake strand, even briefly, while the device is loaded. The autoblock knot - often called the French prusik - is the standard third-hand backup that exists specifically to catch exactly that moment, wrapped around the rope below the rappel device and clipped to the harness, gripping instantly the moment weight comes onto it if the brake hand lets go.
What sets it apart from the classic prusik it's closely related to is how it behaves once someone needs to move again after it's grabbed: a push from a gloved hand releases the autoblock's grip even under some load, where a prusik knot tied directly onto the rope can jam hard enough to need real effort, or even a second tool, to loosen. That difference matters enormously in an actual emergency, which is why the autoblock has become the more commonly taught rappel backup in guiding and rope-access training specifically.
The setup detail that trips up beginners most is where the hitch attaches: it has to sit below the rappel device on the harness side, clipped into a carabiner on the leg loop or belay loop, not simply left hanging free. Attached correctly, it locks the rope against further feed the moment the device stops being actively controlled; set up backwards or unclipped, it provides no backup at all despite looking correctly tied.
Because of how forgiving it is to release under a controlled load, the autoblock has also become a standard component in crevasse-rescue and self-rescue systems more broadly, anywhere a climber needs a hitch that grips instantly under an unplanned load but can still be worked loose by the person relying on it, one-handed, without needing a second tool.
The "French prusik" name reflects its adoption in European mountaineering circles as a specific refinement of the original prusik hitch for exactly this release-friendly behavior, and both names remain in common use today largely depending on which guiding tradition or region taught a given climber. Whichever name is used, the underlying wrapped construction and its easy-release character are identical.
Cord diameter matters more with the autoblock than some climbers expect: cord that's too thin relative to the rappel rope can grip too aggressively and become hard to release smoothly, while cord that's too thick may not wrap enough times to grip reliably at all. Most climbers carry a dedicated length of accessory cord sized specifically for this job rather than repurposing whatever spare cord happens to be on hand, precisely because getting that balance right matters when the backup is genuinely needed in a real emergency rather than a practice session.
How to tie the Autoblock Knot
Take a loop of thin cord and wrap it loosely around both the rappel rope and the carabiner it will be clipped to.
Strength & security
Wrapped around the rope rather than tied directly onto it, the autoblock grips firmly the instant it's loaded (such as a hand coming off the brake strand during a rappel) but, unlike a prusik, releases with a simple push once weight comes back onto the rappel device - a real, meaningful difference during an actual rappel-backup emergency.
Cautions: It must be attached below the rappel device, on the harness side, and clipped to the harness rather than left dangling - a common setup mistake that defeats the backup's purpose entirely if the wraps end up on the wrong side of the device.
Alternatives
- Prusik Knot
Grips somewhat more aggressively but is meaningfully harder to release under load - a genuine tradeoff many rappellers weigh against the autoblock's easier release.
- Klemheist Knot
Shares the autoblock's directional grip character but is generally tied on a taut rope for ascending rather than around a rappel device as a backup.
FAQ
Why is the autoblock preferred over a prusik as a rappel backup?
It releases much more easily under a controlled load - a simple push frees it - where a prusik can jam hard enough to need real effort to loosen, a meaningful difference in an actual emergency.
Where does the autoblock attach during a rappel?
On the harness side of the device. Before ever weighting it for real, most instructors have students test it deliberately at the start of a rappel by briefly letting go with the brake hand a few inches off the ground - a cheap way to confirm the wraps and clip-in are actually correct before trusting it higher up.
Does the autoblock work in both directions like a prusik?
It grips most reliably in the direction it's loaded during a rappel; it's used specifically in that one directional context rather than as a general bidirectional friction hitch.
Does cord diameter matter for tying a good autoblock?
Yes - as a rough starting ratio, 5-6mm accessory cord suits a typical 9-11mm single rappel rope; on a fatter double-rope rappel, climbers often size up slightly or add an extra wrap, since the same cord can feel noticeably less grippy on two ropes bundled together.