In European tradition, knives are beveled on both sides of the blade; coming to a point in the middle. In Japanese tradition, however, the bevel is only formed on one side (usually the right-hand side). What is the purpose and/or advantage of this?
First, let’s explain what is meant when talking about knives, single/double sided blades and their cutting edge. The angle of the knife is also referred to as the “bevel.” Most knives have a bevel on both sides, but some traditional Japanese knives have a single bevel, or even differing bevel sizes. Typically, however, when talking about knives that have a traditional bevel and a 10 degree angle, it means on both sides, making the total angle 20 degrees.
Single blade of typical Japanese style knife looks like this:
C and D blade shapes are respectively for the right and left hand chef’s, while blade shape B is a double hollow.
For vegetable knives, the main advantage of the front-side single bevel is that it’s easier to make super-thin cuts. For example, a test of knife skill involves paring daikon radish to remove the peel, then continuing to cut to make a long continuous paper-thin strip, longer than you would be able to make if you just sliced it. See, for example:
People who have mastered this skill can also make remarkably fast matchstick cuts after making the long piece, and the lack of back bevel may reduce the amount of food that sticks to the knife. Some of these single-bevel knives that features a slightly concave backside (urasuki), which may also reduce the amount of adherence of food to the knife.
For crab knives, the honing is on the opposite side to the hand (the back of the knife), which may reduce the amount of shell fragments that could embed themselves into the meat, and probably allows the meat to cut more cleanly.
In practice, though, most Japanese households use dual-beveled knives. Single-bevel is more common in professional kitchens, especially in the Kansai area where fancy cutting techniques are used in presentation to a greater extent than in Tokyo or north Japan.
Knives meant for cutting regular fish or meat are usually, but not always, dual-beveled, the exception again mostly being for professionals. (Usually Yanagiba and Sashimi knives are single-bevel, but only those used in professional kitchens and almost never at home).
Some sources claim that single-bevel edge is more durable, however the opinions are polarised, otherwise that would be an advantage in high volume restaurant use. A thin, well sharpened Japanese blade may chip more easily than a thicker blade.
With practice you can get slightly more precision cuts with a single bevel than with dual-bevel knives, however, they are as easy to use for cooks with average knife skills.
Sharpening techniques for single vs. double edged blades
If you consider a western-style symmetrically sharpened knife, each side of the knife has a certain angle. For example, a typical mid-high end knife (i.e. globals) can be sharpened to 30 degrees inclusive, meaning each side has a 15 degree angle from the knife centre. A smaller angle means a sharper blade and thus better at cutting.
When it is sharpened on the right side, while holding the knife and viewing down on the back of the blade, the left side of the blade will track vertical through bread, cheese, etc. with the portion that has been trimmed off falling to the right – it tracks poorly if used by a left handed person. It takes a lot of honing to change from centre/right/left styles.
If you took the same knife and sharpened it only on one side, one side would be flat (0 degrees) while the other would have a normal 15 degree angle. As a result the knife blade would have a 15 degree inclusive angle instead of 30 degrees for the same base steel, or in other words it’s twice as sharp for the same price. Single-beveled knives also take less time to sharpen since the flat side only requires a small amount of work compared to a regular bevel. The disadvantages are that it takes practice to learn to cut straight with a single-bevel knife, and the knives are specifically left or right handed.
Historically this was probably a means of making the most of the poor quality steels that were all the Japanese could manage with their local resources. They knew that the quality of their steels was bad, which was also the origin of the peculiar folding and jacketing process used to produce katanas. Note that many modern Japanese knives are double beveled and asymmetric, which is more complicated to sharpen but may improve blade durability somewhat compared to a pure single-bevel.