the blade from torsional bending in the transverse direction (transverse spine). Longitudinal stabiliser In addition to the transverse stabiliser, a longitudinal stabiliser is sometimes created in the form of a ridge parallel to the cutting edge and the blade is ground in two areas or bevels, each with different degrees of hollowness or curvature; the area between the back of the blade and the ridge is typically less hollow featuring a larger radius of curvature, also called the "belly", and the area between the ridge and the cutting edge which is more hollow i.e. with a smaller radius of curvature. These two beveled areas have different curvatures and in a well-made razor they transition seamlessly in the ridge (belly) and the cutting edge respectively. Sometimes there are three bevels. The ridge stabilizes the blade against torsional flexing in a direction perpendicular to its longitudinal axis by acting as a lengthwise spine for the blade. The distance between the ridge and the back of the blade is inversely proportional to the hollowness of the blade and is described in fractional terms in ascending steps of as, for example, hollow, hollow, or or (full hollow). Full hollow indicates that the stabilizing ridge is very close to the midsection of the blade and the farthest from the cutting edge compared to the other grades. This is considered the most expensive blade. At the highest end of hollow ground, more hollow than even the grade, is the so-called singing razor, so named because its blade produces a specific resonant tone when plucked, similar to a guitar string, however such use is not recommended as it can distort the cutting edge. Its manufacturing process is so demanding that a full 25% of the blades get rejected as not meeting standards. Stability and sharpness There is a tradeoff between stability and long term blade sharpness. A full hollow ground () blade can keep a very sharp edge even after a great number of honing cycles because of its high degree of hollowness but it is more susceptible to torsional bending because it is thinner. A partially hollow blade ( or for example) cannot sustain the same degree of sharpness for as long, because as the cutting edge erodes it can eventually reach the stabilising ridge faster where there is more material and thus the cutting-edge bevel cannot be maintained without excessive honing of the stabilising ridge