There are a number of other knots that do the same thing, but the butterfly knot tends to work better because it doesn't jam when strained and it's easy to untie.
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Step 1: Where Will It Be Tied?
(For Rope Tackles, tie the Butterfly Knot near the Anchor Point, leaving enough room to apply strain to the line and reserving enough room to tie off the rope tackle.)
Step 2: Form Loop 1
Step 3: Form Loop 2
Step 4: Working With Loop 2
Click here for Knot Tying Terminology.