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.
There are six contact points in total for each hemisphere, which is enough to control the part's six degrees of freedom. Alternatively, an arrangement consists of three hemispheres positioned within a tetrahedral indentation, a V-groove, and a flat area.