The deba knife is a fish knife/cleaver for breaking down and filleting fish. The style is said to have originated around the end of the seventeenth century in Sakai, historically one of the most productive knife-making centres in Japan. Back in the early Edo era this knife was just as likely to be used on a variety birds and tortoises; the modern deba is primarily seen as a knife for fish and crustaceans that can also be useful for poultry.
Blade sizes range from the ko-deba at as little as nine centimetres or so through to great heavy monsters of thirty centimetres and longer. Professional-grade deba knives are kataha, which means single grind; left-handed and very large single bevel knives commonly require a custom order. Ryoba (double bevel) deba knives are easy to find and easier to sharpen.
The heft of the deba makes it easy to cut through fish bone that would prove stubborn or even damaging for a lighter blade. The deba blade is up to 1cm or more in thickness at the spine, and can perform as an effective cleaver without the swing - set the edge to the fish and give the spine a solid thump with the heel of your hand for a safe, controlled cut. Where the blade thins toward the tip it combines with the bevel to make a very fine point, ideal for precise filleting.