Knife sharpeners need to be able to create an exact angle on the knife's edge. American and European knife blades use a 20-degree angle out of the factory, as Chef's Choice explains, while Asian knife blades use a 15-degree angle. Hence, many people prefer a knife sharpening machine to a simple manual stone, because the machine helps guide you to maintain the proper angle on the blade.
The most advanced hybrid yet with acclaimed Criss-Cross The most advanced hybrid yet with acclaimed Criss-Cross sharpening technology. Electric and manual sharpening stages that provide superior edge geometry and razor sharp edges on both straight edge and serrated knives including household and kitchen knives sports knives and pocket knives. The diamond abrasive wheels sharpen simultaneously "into" and "out-of" ...  More + Product Details Close
This is an important but often confusing aspect of the sharpening process. When you sharpen knives, especially on coarser stones, you'll notice a burr form on the opposite side of the edge. It can be difficult to see, but easy to feel. Carefully feel for the burr by running your finger from the spine of the knife to the edge. The burr will jump from side to side as you sharpen each edge, and once you've felt the burr move to both sides, you can move to the next finer stone. Once you get to the finest grit, the burr will become smaller and smaller!

Whetstones may be natural or artificial stones. Artificial stones usually come in the form of a bonded abrasive composed of a ceramic such as silicon carbide (carborundum) or of aluminium oxide (corundum). Bonded abrasives provide a faster cutting action than natural stones. They are commonly available as a double-sided block with a coarse grit on one side and a fine grit on the other enabling one stone to satisfy the basic requirements of sharpening. Some shapes are designed for specific purposes such as sharpening scythes, drills or serrations.[9]