The two half-hitches is a type of knot

Knot

A knot is a method of fastening or securing linear material such as rope by tying or interweaving. It may consist of a length of one or several segments of rope, string, webbing, twine, strap, or even chain interwoven such that the line can bind to itself or to some other object—the "load"...

, specifically a binding knot or hitch knot. It consists of an overhand knot

Overhand knot

The overhand knot is one of the most fundamental knots and forms the basis of many others including the simple noose, overhand loop, angler's loop, reef knot, fisherman's knot and water knot. The overhand knot is very secure, to the point of jamming badly. It should be used if the knot is...

 tied around a post, followed by a half-hitch. Equivalently, it consists of a half-turn around a post followed by a clove hitch

Clove hitch

A clove hitch is a type of knot. Along with the bowline and the sheet bend, it is often considered one of the most essential knots. A clove hitch is two successive half-hitches around an object. It is most effectively used as a crossing knot. It can be used as a binding knot, but is not...

 of the running end around the standing part.

This knot is also sometimes referred to as a clove hitch over itself.

The following three-step process for tying the two half-hitches is also explained in the image gallery below. Click on the images for high-resolution versions.

1. Begin by forming a clockwise loop around the pole, with the working end of the rope
   Rope
   A rope is a length of fibres, twisted or braided together to improve strength for pulling and connecting. It has tensile strength but is too flexible to provide compressive strength...
   
    on top. Bring the working end through the loop. At this point, you have an overhand knot around the pole.
2. Bring the working end down and to the left. Loop it under the standing end. Pull the working end through the loop just formed, tighten, and slide the knot along the standing end up to the post.
3. A correctly tied two half hitches resembles a clove hitch
   Clove hitch
   A clove hitch is a type of knot. Along with the bowline and the sheet bend, it is often considered one of the most essential knots. A clove hitch is two successive half-hitches around an object. It is most effectively used as a crossing knot. It can be used as a binding knot, but is not...
   
    tied around the standing end of the line, not a cow hitch
   Cow hitch
   The cow hitch is a hitch knot used to attach a rope to an object. The cow hitch comprises a pair of half-hitches tied in opposing directions, as compared to the clove hitch in which the half-hitches are tied in the same direction. It has several variations and is known under a variety of names...
   
   .

To release the knot, pry apart the two hitches with a bending motion. However, it can often be difficult to untie. To help avoid this problem, tie a slipped variation: in the second half-hitch, pass through a bight

Bight (knot)

In knot tying, a bight is a curved section, slack part, or loop between the two ends of a rope, string, or yarn. The term is also used in a more specific way when describing Turk's head knots, indicating how many repetitions of braiding are made in the circuit of a given knot.-Slipped knot:In order...

, as when tying your shoe, rather than the entire free end.