The clove hitch is a fast temporary knot for tying a line off to a spar (post). The clove hitch requires constant tension on the line in order to secure the knot. The rocking motion on a bow line secured to a dock post soon causes a clove hitch to untie. Use the clove hitch in limited circumstances.
A technique is to add a half hitch after the clove hitch has been tied for additional security.