Clove Hitch (video tutorial)
The clove hitch is used most commonly to tie your fenders onto your boat. It is easy adjustable, since feeding in rope from either direction will loosen the knot to be tightened at a new position.
To tie a clove hitch at the end of a rope pass the end around the rail starting at the right, going all the way around the rail and then crossing over itself to make the 2nd turn. On the 2nd turn as you line comes back up put it under the start of the second loop.
Check that both ends are in the middle, emerging in opposite directions. Pull to tighten. When pulled tighter, the rope passing over itself binds it in place.