Now if we were to get into a discussion of the definition of a "fence"...
The last post makes me think of this:
Install five fenceposts. One at 12 O'clock, one at 3 O'clock, one at 6 O'clock, one at 9 O'clock, and one at the center of the field.
The first "fence" is a strand of barbed wire 10 cm above the ground that starts at 9 O'clock, runs to the center where it takes a right angled turn and ends at 6 O'clock. Length = D.
The second "fence" is a strand of barbed wire 20 cm above the ground that starts at 6 O'clock, runs to the center where it takes a right angled turn and ends at 3 O'clock. Length = D.
The third "fence" is a strand of barbed wire 30 cm above the ground that starts at 3 O'clock, runs to the center where it takes a right angled turn and ends at 12 O'clock. Length = D.
In a two-dimensional view it looks like two perpendicular lines at right angles intersecting at the center.