Also known as the Rolling Hitch or Magner's or Magnus Hitch.
The Tautline Hitch is an adjustable knot that is commonly used for tensioning guy lines, such as on a tent. The knot can slide along the standing part. When the knot is released the tension on the standing part tightens the coils in the knot, increasing the friction which thus keeps the knot in place under tension. Since the tautline Hitch is tied to its own standing part, the rope must be able to slide around the object to which it is secured for it to be adjustable. This knot is known as the Rolling Hitch when, instead of being tied around its standing part, it is tied around another rope, spar or cylindrical object.
Place the line around a secure object and bring the working end under the standing part. Wrap the end around the standing part twice, working toward the secure object. Now take the end back beyond the two wrappings (away from the secure object) and add a half hitch.
Tighten the loops. Be careful to keep the knot neat whilst doing this and tighten it only enough so that it will grip the standing part reliably, but not so tight that it cannot slide easily.