HSS is a high tungsten (18%) grade of T1-HSS . HSS is recommended for softwood and some occasional hardwood use. All knives are double edged and fit only the Tersa brand cutter heads. We offer these knives in the original Swiss Made Tersa brand knife and the German Made Tigra Brand Replacement, Each meet stringent qualtiy control. The Tigra are exceptional, they have a black oxide coating to prevent oxidation. The tersa oem does not offer this coating. The Tersa Brand is also more expensive.