Taut Line Hitch
Purpose: an adjustable loop knot for use on lines under tension. It is useful when the length of a line will need to be periodically adjusted in order to maintain tension.
Tying: Pass the end around the main rope (or the standing end of itself) to make a Half Hitch. Continue around going over the first turn. Tuck the rope between the standing end and the first turn. Tighten to make it secure (this introduces a slight dog-leg in the main rope). Continue around to add a final Half Hitch.
Animated Knots: http://www.animatedknots.com/rollinghitch/