When knifemakers tell someone that they should put a 20-degree angle on their knife, it means that they should sharpen each side to 20 degrees. This makes a total angle of 40 degrees. So, talking about the angle on a knife, is talking about the angle at which you should hold the knife to the sharpening stone.
Under 10 degrees
The lowest angles are for edges that are typically cutting softer materials. The edges should not be subject to abuse so the lower angle can be maintained without damage or edge failure. The lowest angles that we typically see are on straight edge razors. These are sharpened to an angle which is roughly 8 degrees. A straight razor has a very delicate edge and are very easy to damage. With proper usage, a straight razor should never see the type of use that would damage the edge.
10 to 17 degrees
A sharpening angle of 10 to 17 degrees is still very low for most knives. With a total of 20 to 34 degrees, this still makes a very fine edge. This edge is often too weak for a knife that might be used in any type of chopping motion. Also consider that harder steels are also more susceptible to impact damage because they are more brittle. If the knife is used for cutting soft items or slicing meat, a lower angle can hold up and provide a very smooth cutting action.
17 to 22 degrees
Most kitchen knives a sharpened in 17 – 20 degrees. Some, often Japanese knife makers, will sharpen their knives to about 17 degrees. Most standard knives are sharpened to 20 degrees. Most people will experience that kitchen knives sharpened to 15 to 20 degrees cut very well and are still durable. These thin angles are still not as durable since an angle under 40 degrees will not respond well to rougher treatment in harder materials.
22 to 30 degrees
In this range, the knife edges are much more durable. A pocket knife or a hunting knife will inevitably see abuse not seen by knives meant primarily for slicing or chopping softer materials. While the edge may not ultimately cut as well (but you may not notice a difference) it will be considerably more durable.
Over 30 degrees
Any edged tool or knife that is sharpened past 30 degrees will be very durable. Because of this its cutting ability will be reduced. The durability has an advantage since more force can be used to make the cut. While many knives won’t benefit from this sharpening angle, an edged tool like a machete, cleaver or axe must be very durable as the typical cutting action of these tools would damage thinner edges.
*written by William Drake, edited by Thomas Juhler