Anatomy of the DE
The first piece of information is to define the proper terms for each piece of a Safety razor. A basic Merkur DE razor was used as a model but this model applies to any DE razor.
Note that the cap is curved more at the top than at the bottom. In other words, the radius is smaller at the top than at the bottom. The edges of the guard lie on the same circle as the top of the cap. The blade itself conforms to the shape of the bottom of the cap. The blade's edges seem to lie on the line connecting the cap edge with the guard edge.
Finding the correct angle
It is often stated that a good blade angle to start out with is about 30 degrees (relative to the skin). In order to find the corresponding handle orientation, it is recommended to place the top of the razor against the cheek with the handle being perpendicular to the surface of the skin, then lowering the handle until the blade just touches the skin. See the diagram on the left.
Note that for this razor's geometry, that angle can also be obtained by placing the guard against the cheek with the handle pointing to the floor, then raising it until the blade touches the skin. This is because the edges of the guard, the top cap, and the blade all lie roughly on the same line. See the diagram on the right:
Please keep in mind this approach is only valid if no pressure is applied to the skin. If that is the case, depressions are formed, and it can results in nicks and cuts.
Let's examine what happens during a "perfect" stroke. When both the cap and guard edges touch the skin, the blade is also in contact and will cut whiskers very close to the skin.
Obviously, the razor was designed precisely so that the blade edge just touches the skin. If the handle is held such that either only the guard edge or only the cap edge touches the skin, the blade is lifted away from the skin. In this case, the shave won't be very close. That's why it is so important to figure out the curvature of one's face, so that the handle can be oriented properly. This is very difficult to achieve consistently, so don't despair too much, help is on the way.
How to overcome the difficulties of maintaining proper curvature
Essentially three options are available for the DE shaver (please let me know if there are others):
1. Apply pressure. Using more pressure allows the blade to cut the skin at other angles than those defined by the razor geometry. This method is difficult to master, however, we all use it occasionally, albeit perhaps involuntarily. Often, irritations, nicks or cuts are the result. This option is not recommended.
2. Use a Slant. The twisted blade will cut the hair at a 45 degree angle (blade vs. hair, not blade vs. skin). The slant is used like any other DE razor with the same blade angle.
3. Use adjustable razors (see below).
The adjustable razor
For adjustable razors, the distance (along the handle axis) between the guard and the blade can be varied. I don't know all adjustables on the market, but I surmise that the geometry of the razor at the lowest setting corresponds to that of a regular, non-adjustable razor as described above. When the distance is increased, there are now two options available for the handle orientation.
Orientation 1: In this orientation, the top cap is in contact with the skin, and the blade angle (with respect to the skin) can vary from about 30 degrees to somewhat larger numbers. See diagram on Right.
Orientation 2: In this orientation, the guard is in contact with the skin, and the blade angle (with respect to the skin) can vary from large values (about 60 degrees) to about 30 degrees. See diagram on left.
The larger angles result in a more aggressive disposition of the blade. Therefore, using the top cap as a guide is safer than using the guard as a guide. Most DE users would instinctively place the guard against the skin and then vary the angle, rather than using the top as a guide.
Using an adjustable razor at settings other than the lowest one, will likely result in closer shaves, but also in more irritations and perhaps more nicks and cuts because of the steeper blade angle.
What about the blade being parallel to the skin?
It is sometimes assumed that a parallel orientation of the blade (with respect to the skin) would be optimal. However, no DE or SE razor actually allows the blade to come close to the skin when it is held parallel.
The top cap will touch the skin first, resulting in a large gap between the skin and the blade. The ideal of a parallel blade is an exclusive of the straight-razor world.
More picturesNick Stevens (doesnotcompute) agreed to share his pictures in Wiki which is great. A big thank you to Nick.
The following pictures should give any user an idea of the amount of blade angle difference that can be achieved with a Merkur 34c/HD.
Description Picture Clearly, if you shave with the blade at this angle, you are going to be a long time waiting for that BBS you dreamed of Keep dreaming but notice the difference in angle of the razor handle If you have a beard an inch long, you might hack away at some of that hair, but you couldn't call it shaving If you went a week without a shave, you might now be trimming the tips of those hairs Now we are shaving! Notice how the angle of the blade is quite steep against the skin. This is sure to result in nicks or cuts if you apply too much pressure. Even if you don't cut yourself, you'll end up scraping the skin causing irritation The blade angle here is much shallower than in photo 5, which means only the very point of the blade edge is going to slice through the hairs and hopefully glide over the skin. Now we're going too far the other way, just trimming the tips of the longest hairs We're not chopping trees you know