The figure-8 follow-through (sometimes "rewoven figure-8" or "trace-eight") is the standard climbing tie-in knot. The climber ties a figure-8 knot in the rope, threads the working end through both tie-in points of their harness, and then "follows the rope back through" the original figure-8, mirroring its path. The result is a strong, easily-inspected knot that ties the climber to the rope.
Why this knot? Two reasons. First, it is easy to inspect at a glance β both legs of the rope follow the same figure-8 path, so any partner can verify the knot is correctly tied in two seconds. Second, it tightens under load and stays put. The figure-8 follow-through has caught millions of climbing falls without slipping.
The knot is taught in every belay course at every climbing gym worldwide. Always tie in through both the waistbelt and leg-loop tie-in points (never the belay loop). Always finish with a stopper knot β most gyms require this. And always do a partner check before climbing: harness, knot, belay device, locked carabiner.