UPDATED! The Blade-Bar Span and the Challenge of Measurement

[This article has been updated at about noon EDT on 11Mar2015 to reflect more standard terminology. Unfortunately, the terminology in my earlier articles will have to stand as written and as defined in those earlier articles.]

The razor-design aspect that is usually referred to as the "span" is the blade-bar span, or, perhaps more precisely, the guard span. I'm just going to call it the "span."

Although in most DE razors with a safety bar there is some kind of visible open space between the blade edge and the safety bar, this open space has little to do with a razor's shaving character. This is not the blade-bar span that influences the shaving behavior of the razor. I will refer to this relatively-easily-measured opening the blade-bar gap, the blade-guard gap, or simply the gap.

The Lord L.6 razor head shows the large potential difference between the blade-guard span and gap. The A on this photo labels the blade edge, which is the ONLY hard point that can be used when measuring span. Point C (in a light color on the dark baseplate) represents the other second measurement hard point when measuring blade-guard gap, which is often mistaken for the blade-bar span (and gap measurements aren't useful for understanding shaving character of a razor).  Point B, where the shave plane actually touches the baseplate, represents the location of the ISPDL, and is not a hard measuring point. Because of this lack of second measurement hard point, measurement devices cannot readily, accurately measure the span, A-B, directly from the razor, and are likely best done from a photo. This view highlights the potentially huge difference between gap and span measurements. So beware: published gap measurements are likely to provide little useful information for understanding or comparing a razor's shaving character.

Like the blade exposure, which is a measurement between a hard point (the blade edge) and an imaginary point (the shave plane), the blade span is also a measurement between a hard point (the blade edge) and an imaginary point (the imaginary line along the safety bar where it touches the shave plane; and I will refer to this line as the ISPDL -- the Imaginary-Shave-Plane-Determining Line).

This is a very important point.

The gap, which is between two hard points on the razor, may be similar to a given razor's span, but is almost always different. The gap measurement isn't useful to compare one razor to another either because the degree of variation between gap and span measurement on a given razor may be large, will also vary greatly from one razor design to another.

So to be somewhat redundant: I can use some kind of instrument to measure the blade-bar gap, and I may be able to do that very accurately right on the actual razor. However, despite the accuracy of this measurement, it isn't necessarily a measurement of the blade-guard span. As a result, it (the gap) likely will not explain the shaving behavior of the instrument.

Take the Lord L.6 razor head for example. This is the razor that months ago caused the light bulb to go on over my head, when I recognized the difference between blade-guard span and gap. Because of the cross-section design of its safety bar, there is a large difference between the two measurements. The cross section of the L.6 is such that it slopes inward from the ISPDL down toward the actual hard point that defines the blade-guard gap.

Because the blade-guard span has only one physical hard point against which to measure (that is, the blade edge) and its other measuring limit is an imaginary line along the highest point of the safety-bar curve (that is, the ISPDL, the line along the safety bar that first touches the face when the razor is in its optimal shaving position against the plane of the cheek), this cannot be accurately done on the razor. The span is perhaps best measured on an enlarged side view of the razor with the shave plane drawn in. Its distance is from the razor edge to the ISPDL.

For emphasis:

The span is always measured along the shave plane (pretty much -- depending on blade exposure). This cannot be done accurately on the razor itself with any measuring device. At best, on-razor measurements of the span are only approximations, which may vary greatly in validity and reliability.

The ultimate conclusion is that, given a better understanding of the difference between blade-bar gap and span,  gap measurements are not terribly helpful to understand the absolute or relative performance of a razor; it is the span that is much more significant, though much more difficult to obtain.

Happy shaving -- and measuring!