the fit between the foot and the part of the shoe rear of the hinge line never varies the invention is a sole structure with the above defined hinge line and hinge means with a horizontal length equal to the vertical thickness of the sole sections the vertical thickness is defined to be the shortest distance between the top surface and the bottom surface of a sole section this last characteristic was found to be necessary to enable a completely flat shoe sole to work with a hinged sole assembly if the length of the hinge means is greater than the thickness of the sole a wearer will feel the hinge if the length of the hinge means is less than the thickness of the sole it will not flex properly if used with a thin sole structure because a large separation will occur see fig1 between the rear edge 102 of the lower surface of the forward sole section and the forward edge 103 of the lower surface of the rear sole section thus the claim language specifying that the length of the hinge means should be equal to the sole thickness makes the invention applicable to thin sole structures such a condition can apply to a sole structure that is not thin but it is not possible to make a comfortable flexible thin soled structure that does not have this condition the prior art examples of ghez and kapskobund have notches and grooves to facilitate bending because their hinge dimensions have not been matched to their sole thicknesses such designs can not be made as thin as would be required to make a working boot design in.