In Japanese knife making blades can be made from; 1,2 or 3 layers of steel to achieve different results.
Most Japanese kitchen knives here are made of 3 layers, call San mai. The process includes welding softer steel on the outside to the hard core steel. The Core is what makes up the edge of the knife and when we refer to the steel type this is what we are referring to. The steel on the outside is referred to as cladding. In many cases the knives made of carbon steel, the kind that can rust, have both carbon steel cladding and carbon steel core. Likewise, in a stainless knife the cladding and the core are both stainless steel. On some occasions the blacksmith will use stainless steel as the cladding on a carbon steel core. This hybrid type will allow the user the awesome performance of carbon steel with less concern of the knife rusting. Only the exposed carbon steel at the edge can rust easily and still needs to be kept dry.
2 layered knives are made with one piece of hard steel and one of soft steel. The single bevel knives that have quite specific purposes and are primarily designed for Sushi making. Their design allows for super fine slicing of fish and vegetables or the Japanese techniques if fish fileting. The are awesome but feel quite different to use.
Single steel knives make very precise, fine edged knives. They are incredibly tricky to make by hand and called “Honyaki” meaning true forged. These are some of the finest knives available. These can also be made in a factory by rolling out a blade thinly to achieve similar results.