Carbide is a very hard metal that can increase tool life up to 12 times that of HSS. It is recommended for hickory, teak, tropical dense hardwoods, MDF, edge glued stock and other man made materials. Carbide can nick easier so it is to be used with clean wood. Consider running two carbide knives and two HSS knives for long life and smoother finishes. All knives are double edged and fit the Tersa™ brand cutterheads. These knives are the original Swiss Made Tersa™ brand knives.