A hex key (Allen wrench) is a specialized tool shaped like an L and used for hex socket head screws. It has a hexagonal rod shape, which is simple and lightweight, specifically designed for turning internal hexagon screws. The size of a hex key is determined by the flat-to-flat dimension of the hexagon. Bolt sizes follow national standards, and some models have a ball at the end, allowing the tool to be used at an angle to the screw. The increase in angle allowed by the hex key reduces the neck size and correspondingly decreases its strength. Therefore, the size of the ball-end neck affects the allowable angle and strength. Generally, hex keys from reputable brands are made of Cr-V steel or Cr-Mo alloy steel, while higher-quality ones are often crafted from S2 premium alloy steel, with a hardness that can reach HRC60±2.