Here is a figure I came up with after studying the tetrahelix.
This must have been some time before November 2, 1981, since
that's when I started to analyze its geometry mathematically.
It is fairly easy to construct with equal-diameter spheres or
toothpicks. The tetrahelix is based on a figure of two tangent
spheres which form the center of an arc of four other tangent spheres.
In toothpick terminology, this is *three* face-bonded tetrahedra sharing a
common edge. I wondered what I would get based on a figure with
*four* face-bonded tetrahedra and this is what came out.

I call it a double tetrahelix since it can be interpreted as two
tetrahelices bonded together. They share
two common strands of spheres which are tangent all the way along
the strand. Each helix has an unshared strand of spheres which
has small gaps in it. The gap corresponds to Buckminster Fuller's
"Unzipping Angle" (2π - 5⋅acos(1/3), or about 7.35610°)
which is discussed in Section 934.00 of *Synergetics*.
There and in the previous section (933.00) discussing the tetrahelix,
Fuller draws a tantalizing correspondence between DNA and the tetrahelix,
though I've never seen a double tetrahelix model constructed by him.
It is an interesting and natural conjecture, but as yet I haven't seen a
biochemist structurally verify this or describe any correspondence
between the two. Certainly biochemists have shown a lot of interest
in geodesics and tensegrity. This would be another good place for them to dip
their oars into Fuller's work.

The "Double Tetrahelix" figure available in the Tensegrity Viewer is a "toothpick" version of this model. The green backbones of that figure have slightly more than unit-length segments reflecting the gap, while all the other segments are unit length. There is also a VRML model which tries to show the correspondence between the sphere model and a toothpick model.