The extensions in this 4 pc set feature the largest knurling surface, offering the most control, each extension has knurling on the female drive end to help when hand-tightening fasteners or working with oily materials. Additionally, select extensions in the set have knurling on the shaft for more control. The set includes a durable compact storage rack designed to optimize drawer space for long-term use. Chrome plating offers corrosion and rust protection. All Milwaukee mechanics hand tools are backed by a lifetime guarantee.