Some Japanese knives are sharpened on both sides of the cutting edge, some only on one. When sharpened only on one edge, it’s called “kataba” style. Sharpening only on a single edge allows the very thin slices that are an important feature in Japanese food.
Traditionally, high carbon steel (“hagane”) was used in Japan. There are two grades of it, “shiroko” (“white steel”), and a slightly better one, “aoko” (“blue steel”.) The blue steel grade is more resistant to marking and corrosion,
The Japanese use the word “honyaki” to mean knives forged from a single piece of metal. These are very expensive, chip easily if not used properly, and hard to sharpen. They are for professionals only.
In contrast, the word “kasumi” is used to designated knives made by joining carbon steel to soft iron. The carbon steel provides the cutting edge of the knife; the soft iron the rest of the knife’s body (except for the wooden handle.) These knives are cheaper and less brittle.
Knives are of course also made from stainless steel now in Japan, though professional chefs often still prefer the high carbon steel blades.
Japanese knife handles are traditionally made of wood. Many of the more expensive knives come with wooden sheaths as well to cover and protect the blade.
The two most used knives in Japan are the deba bocho and the nakiri bocho.
Some Japanese knife makers have been in business 400 to 700 years.