The MILWAUKEE® 1/2 Drive 21MM Metric 6-Point Deep Well Socket with FOUR FLAT™ Sides was designed from the ground up to be part of the most versatile socket family. The 1/2” drive sockets feature an innovative design with four parallel flat sides, which deter rolling and are wrench-compatible. The metric socket sizes are stamped into the flats providing improved, easy-to-read visibility. This deep well socket has been optimized geometry to reduce bolt rounding and stripping. Milwaukee backs their 1/2” Drive Metric Sockets with a lifetime guarantee.