The illustration above was posted recently in the DE razors forum (B&B URL) and the general shaving discussion forum (B&B URL), but since the illustrated safety razor parameters and modes of shaving apply both to DE and SE safety razors, the illustration seems to belong here, too.
The comprehensive illustration shows the blade and shave planes and the parameters that follow around the blade cutting edge for a blade loaded in a safety razor: blade angle, handle angle, blade exposure, guard span, cap span, blade gap, free-end distance, clamp distance. Free-end and clamp distances were measured for the first time with my photo analysis of the Edwin Jagger (EJ) DE89 head (B&B URL). Important details that could not be simply illustrated are included at the bottom of the picture. For example, I rigorously define blade gap based on personal experience in measuring the quantity with my accurate set of micrometer-measured feeler gauge blade combinations (B&B URL).
Blade gap is commonly associated with razor aggressiveness, but, at best, blade gap only loosely relates to razor performance. Blade exposure, guard span, and blade angle, on the other hand, are very important performance parameters with respect to aggressiveness and efficiency. Unfortunately, these three parameters are not easily measured. Photo analysis is the primary method for measuring the illustrated parameters, but physical measurements, such as for blade gap, should be used when appropriate to complement or supersede digital measurements.
Steep- and shallow-angle shaving are common terms (often employed without hyphens), but based on my Internet search, it appears that I am the first to use the term "neutral-angle shaving" (with or without the hyphen) and, within the context of shaving, the term "neutral blade angle". Neutral-angle shaving favors neither the guard nor the cap, justifying the use of the word "neutral", while steep-angle shaving favors the guard and shallow-angle shaving favors the cap. Accordingly, we have neutral, steep, and shallow blade angles. I encourage the use of these terms.
It is hoped that this picture or an updated version will become the standard reference illustration for safety razor parameters with respect to neutral-, steep-, and shallow-angle shaving.
Feedback is welcome. What do you think?