Abstract: It is often desirable to keep a pair of modified involute spur gears, generated with rack cutter and with unequal coefficients of cutter shifting, in mesh with adequate clearance and without backlash; therefore their addendums must be reduced to definite amounts. In this paper, although this fact is proved both mathematically and geometrically; but the emphasis is made to -the later one of the two methods. In the geometrical method, it is shown by the aid of a diagramatical sketch for explaining, the positions of the mating gears relative to each other arid to their rack cutter are clearly drawn. For is simplicity and obviousness this method is much better than other.