Each hemisphere has two contact points for a total of six contact points, enough to constrain all six of the part's degrees of freedom. An alternative design consists of three hemispheres on one part that fit respectively into a tetrahedral dent, a v-groove, and a flat.
Each hemisphere has two connection points, totaling six connection points, sufficient to limit all six degrees of freedom. An alternative design includes three hemispheres on one part that align with a tetrahedral indentation, a V-groove, and a flat surface.