The Thief knot looks similar to the Reef knot, but the running ends are on the opposite sides of the knot. It’s one of the three Reef knot twins, very insecure one: it comes loose under the smallest strain. Use it for everyday household purposes only.
Fold the rope to form a loop with the running end downside.
Pass the 2nd running end through the loop under the 1st running end.
Pass the 2nd running end over both ends of the 1st rope.
Pass the 2nd running end under the 1st standing part and through the loop.
Tighten the knot.
The Thief knot is complete.