The fence corner in this picture has been in place for eight years and, according to some experts, this corner-post style is stronger than any of the common three-post corners.
Strength is measured not by the size of the corner post but by its resistance to heaving out of the ground. The secret is the triangular bracing, and the key to its strength is that the braces are not anchored but are instead floating on a flat rock or plank. This allows the corner post to rise out of its hole during frost heaves without tearing apart the whole corner setup.
The triangular bracing is formed by the vertical post and two braces with several wraps of high-tensile fence wire running from the bottom of the post to the end of the brace. A twist stick (rebar works well) tightens the wire. To determine the length of the angle brace, measure the vertical post from its base to the top wire, and double that length. Then measure along the top wire horizontally from the vertical post, and set your flat rock at that point on the ground. The angled top of the brace connects to the vertical post, just below the first line of wire.