This is a reliable and useful knot for attaching a rope to a poll, post, ring, etc. It is exactly what it's name implies. One half hitch and then another tied on top of the first providing considerably more security than a single half hitch. Two half hitches form a loop that closes and tightens the more you pull. This knot can slip apart under high stress loads.
To Tie Two Half Hitches:
Loop the end of the rope to be tied off around the object and under the standing part of the rope. Take the same piece of working rope and pass it down through the loop you have just created.
Repeat step one and two so that two complete Half Hitches sit snugly on top of each other.