Constrictor Knot

The constrictor makes a very secure hitch. Formed from a clove hitch with the working end secured by a trapping hitch.

Start by tying a basic clove hitch.
Take a full turn around the post / rail, being sure to lay the working end across the standing part.
Thread the working end through under the diagonal but over the standing part. Then bring the working end back, this time to pass under the standing part as indicated by the arrow.
Work the hitch tight so that all the turns sit snugly together and the tip of the working end trapped beneath the loop nearest to the standing part of the rope.