[Jeff]

If the stationary head must be precisely centralized between the feet, and the hinge pin for the pivot center's rotation must be a vertical line drawn from the top of the stationary head to the ground, then the hinge pin line will simply be a vertical line drawn precisely between the feet with the pivot center located on that line at address.

That's why I think that most golfers (like Tiger Woods) do not have a pivot center in their swing.

Jeff.