An adjustable hitch may be any loop knot used to tie a rope around an object that allows the length of the line to be adjusted while under tension. The Taut-Line Hitch is the most common knot of this type and is often referred to as an adjustable hitch.
The Tautline Hitch (also called the Midshipman's Hitch, Magnus Hitch and Rolling Hitch) is an adjustable loop knot for use on lines under tension. It is a useful knot when the length of a line will need to be periodically adjusted in order to maintain tension. Tension is maintained by sliding the hitch to adjust the size of the loop, thus changing the effective length of the standing part without retying the knot. A versatile knot, it can be used for securing tent lines, when climbing tree's, and to secure loads on vehicles just to name a few.
To Tie a Tautline Hitch:
First, pass the free end through the inside loop twice.
Then pass the end outside of the loop, but around the rope with tension.
Pull to tighten. You can now slide the knot along the main length of rope to tighten or loosen the line, as needed.