This chapter provides the definition of the gear parameter limits for the spur and helical gears with asymmetric teeth. It describes the boundaries of the asymmetric involute gearing parameters. The maximum number of gear teeth is limited by application practicality and manufacturing technology. The minimal numbers of teeth of the internal gear mesh are limited by the "tip-tip" interference condition. Application of gears with asymmetric teeth allows the drive flank pressure angle to be increased in comparison to gears with symmetric teeth because of the coast flank pressure angle reduction. A gear tooth size is a main parameter in the definition of root bending stress. Some computer numerical control gear hobbing machines can produce gear with number of teeth up to 1,000 using one-start hobs. Spur reversible asymmetric gears can transmit rotational motion and load by both drive and coast tooth flanks. Direct Gear Design significantly expands boundaries of involute gearing.