I have need to mount two sockets on a rod, perpendicular to each other.

This thing will slide onto a 5/16" rod (or 8mm rod), and give you two ball socket connection points.

Very handy if you want to be able to form a shelf on a tetrahedron with ease.

1) Print a few of these out
2) Slide them onto rods
3) Push spherical capped rods into the sockets
4) Rejoice!