1. We have demonstrated the measuring facilities provided by differential mechanisms, their structural and kinematic unity, and their degeneration into parallelogram devices which constitute a double tangential mechanism. 2. The reason for the elimination of the effect of the component's run-out on the measurement results in differential mechanisms has been demonstrated analytically. 3. Formulas have been suggested for determining the displacement of the indicator stem in terms of the depth of machining and the geometrical parameters of the mechanisms. 4. The differential-parallelogram nature of S. A. Mazin's measuring mechanism has been demonstrated.