The knot starts out like a clove hitch. One turn around the spar and over the standing end and back over the spar. You can do this either to the left or the right.
Then take the bitter end and pass it under the start of the first turn.
I use this for securing items to railing or life lines. Even fenders secured this way on small life lines will not slip. Quick, easy and secure.
Finally, below are some variations of this hitch.