TWIN Knife Manufacturing: SIGMAFORGE SIGMAFORGE is one piece precision forging used in manufacturing several ZWILLING J.A. HENCKELS TWIN knife series including TWIN Four Star II, TWIN Select, TWIN Pro "S", TWIN Five Star, and Four Star. SIGMAFORGE knives are forged from a single piece of steel and have the following benefits: Perfect geometry High forging precision Higher stability of the blade Less metal fatigue - improved steel structure Improved cutting edge retention and lasting sharpness Friodur ice hardened - Maximum no stain properties and more blade strength Shown below are examples of SIGMAFORGE knife series; TWIN Pro "S" and TWIN Four Star II TWIN Pro "S"