How to Tie and Use an Autoblock Knot

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The autoblock knot, an easy-to-tie friction knot or hitch that is tied around a climbing rope with a thin length of cord, is used as a safety back-up knot when you’re rappelling. The knot is the best back-up simply because it does two jobs exceedingly well: It locks under load and, unlike all other friction knots, it releases while still under load. 

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When to Use an Autoblock Knot

Autoblock knots are easy to tie and easy to use.
The autoblock knot is an essential safety knot that you should use as a safety back-up knot every time you rappel. Photograph © Stewart M. Green

An Essential Safety Knot for Rappelling

The knot is tied below the rappel device, and it slides down the rope as you rappel. If you stop, the knot tightens and cinches onto the rappel rope. When it cinches, the autoblock knot stops you from rappelling if you let go of the rappel ropes. The autoblock knot is an essential climbing safety knot—one that every climber should know how to tie and use. In Europe, it's called a French Prusik knot.

Use an Autoblock When Rappelling

Rappelling is one of the most dangerous aspects of climbing since you’re relying solely on your equipment, your anchors, and your climbing smarts. It’s important to take every possible safety measure to minimize the risks of rappelling. You double check your rappel device. You double check the anchors your rope is threaded through. And you use an autoblock knot on the rope as a safety backup.

The Autoblock Keeps You in Control

The autoblock knot allows you to safely stop and hang to clear rope snags; toss the rope farther down the cliff; free twists and knots from the rope; keeps you from losing control, especially on free rappels; and stops you if you get hit by a falling rock. The autoblock also allows you to rappel slowly and stay in control, especially on free or overhanging rappels where you are not able to touch the rock.

What You Will Need

To tie an autoblock knot, you need either a short length of thin cord or a nylon sling.

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What You Need to Tie an Autoblock Knot

The thinner the cord, the more it bites on the rope.
You need either a thin cord or nylon sling to tie your autoblock knot. Photograph © Stewart M. Green

Use a Sling for Your Autoblock

Autoblock knots are easy and fast to tie. To tie an autoblock knot, you need either a short length of thin cord or a nylon sling.The knot can, however, be tied in an emergency situation with any piece of cord or webbing you might have on you. I’ve even seen it tied with the cord threaded on a Hexentric nut. Many climbers use a two-foot, shoulder-length, 9/16-inch-wide sling for their autoblock since it’s a common piece of gear that is always carried when climbing. It’s best to use a nylon rather than a Spectra sling. Also, use narrow webbing rather than one-inch-wide webbing.

Use Cord for Your Autoblock

Other climbers use a piece of cord attached to a carabiner that is carried specifically for tying the autoblock. Use a thin cord (best if it's 5mm or 6mm in diameter). You’ll need a 48-inch length of cord to make this loop. The finished length should be 18 inches long after the ends are tied together with a double fisherman’s knot forming a closed loop.

Remember that the thinner the cord, the greater the bite it will have on the rappel rope but the faster it will wear out. Also remember that since this cord is meant to be loaded, it is possible for the double fisherman's knot to lose its tails, that is the tail can slip into the knot, and it can come undone. Always make sure you have two-inch tails on the knot. Tape the tails to the cord and you will see if slippage occurs.

Check the Cord for Wear

It’s very important that you regularly check your autoblock sling or cord for wear and tear. Look at it after every long rappel to make sure it’s not getting too worn. Look for stitching starting to unravel on sewn slings and for wear from sliding down the rope. When it’s looking worn, retire it and use a new one.

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Step 1: How to Tie an Autoblock Knot

Make sure the knot is on the outside of the knot.
First, wrap the cord or sling several times around the rappel rope. Photograph © Stewart M. Green

The first step to tie an autoblock knot is to clip a carabiner, preferably a locking one, onto the leg loop of your harness. Clip it on the side where your brake hand will be.

Wrap the Cord Around the Rope

Next, wrap your autoblock cord four or five times around the rappel ropes.

More Wraps Equal More Friction

Use up most of the cord on the wraps. How many wraps you put on is up to you, but the more wraps, the more friction. If you don’t use enough wraps, the autoblock will slip on the ropes, particularly if they are new and slippery. If you use too many wraps, the knot won’t slide easily. Make sure that the cord’s knot or the sewn overlap on the sling is not in the knot itself on the rope, but rather on the outside of the knot as in the photograph above.

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Step 2: How to Tie an Autoblock Knot

Finish tying the autoblock knot by clipping both ends into a locking carabiner. Photograph © Stewart M. Green

The second step to tie an autoblock knot, after wrapping the cord around the rappel ropes, is to clip both ends of the cord into the locking carabiner on your harness leg loop. Then lock the carabiner so the cord can’t come undone from it. Finally, dress the knot by arranging all the wraps so they’re neat and not crossed. Make sure the knot is not tightened or cinched down on the ropes so that it slides easily as you rappel.

Make Sure The Knot Won’t Jam

It’s very important to check the knot before you use it by making sure that the length of cord or sling is not too long after being tied to the rappel ropes.

  • First, check that your autoblock knot cinches onto the rope when it’s weighted.
  • Next, check that the sling is not so long that it will jam in the rappel device when it’s weighted. If you’re using a short sling that is no longer than two feet then it shouldn’t be a problem if your rappel device is attached to your belay loop and the knot is attached to your leg loop. Double check to make sure though. If the sling or cord is too long and it jams in the rappel device then it will lead to all kinds of problems when you’re rappelling. If it is too long, then use a sling on your belay loop to extend the distance that the rappel device is from your harness and the knot.
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How to Use an Autoblock Knot

Use either your guide hand or brake hand on the autoblock knot.
Here's how your autoblock knot and rappel device should look when you're ready to rappel. Photograph © Stewart M. Green

You’ve threaded the rappel ropes through your device, tied the autoblock knot and attached it to a carabiner on your leg loop. You’re now ready to rappel with the autoblock as a safety back-up.

Two Ways to Hold the Knot

Before you rappel, make sure the autoblock is loose on the ropes so that it slides easily. Put your brake hand, the one that keeps you in control, below the autoblock knot and grasping the rappel ropes. Put your guide hand on top of the knot below the rappel device and begin rappelling. Or place your brake hand on the knot and use your guide hand above the device. Either way, works fine. Try it both ways and decide which you prefer.

Let the Knot Slide on the Ropes

As you rappel, let the knot slide with your hand keeping it loose. If you want to stop, simply let go of the knot and let it cinch onto the ropes. Make sure you let go of the knot if you need to stop. Novices have died by gripping the knot, which slips on the rope and melts. Let go and let the knot lock.

Avoid Having Your Knot Jam

Make sure that the cord or sling that forms the autoblock knot is not too long. If it is too long, the knot can jam in your rappel device when you stop, which will cause you all kinds of headaches as you work to free it from the device. Avoid problems by making sure the sling is short enough before rappelling. If it’s too long, tie a knot in the end of the sling to shorten it or extend the rappel device from your harness by attaching it to a sling.

Get in the Habit of Using the Autoblock

Get in the habit of always using the autoblock whenever you rappel. It's used by all climbers in Norway when they rappel, and by guides in Chamonix. You might see it more rarely in the U.S. But since it takes only 30 seconds to tie, it is easy to use and it can save your life.