Abstract: A seam interconnection to relatively thick material parts such as leather parts of shoes comprises parts which have edges which are aligned and have one surface such as the interiors or skin side surface of the leather which is formed with a groove extending along its height which is substantially parallel to and spaced inwardly from its associated edge. The two parts to be joined are aligned back to back with their outer or grain sides engaged and a threaded seam is made to hold the two parts together by placing the seam in the grooves of the respective parts. The parts are then formed to a desired shape, for example, are curved so as to form them into a heel and in so doing, the marginal area adjacent the edge is bent backwardly upon itself so that the overlying part extends into the groove. When a heel portion of a shoe is formed the curved configuration of each part is made so as to complement the configuration of the overlying parts resting in the groove so that an irregular surface is avoided.