Built-in spring tension for easy application to the nut. No sharp corners. Designed for superior performance with steel pin and tension washer construction for added strength, no binding or locked joint. Forged and machined for best fit and unyielding strength. Precision heat-treated and tempered for optimal hardness, ductility and durability. Easy to read stamp pattern in order to quickly determine opening size. Meets or exceeds ASME B107.110. Made in the USA.