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Kinematic coupling describes fixtures designed to exactly kinematically constrain the part in question. A canonical example of a kinematic coupling consists of three radial v-groves in one part that mate with three hemispheres in another, credited to Maxwell. 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. One alternative, favored by Kelvin, consists of three hemispheres on one part that fit respectively into a tetrahedral dent, a v groove, and a flat.