The change in total-to-selective ratio, R _V , over the projected height z of the LOS toward a background star, assuming different values of ﹩{S}_{\max }﹩. Two dust-to-gas ratios of η = 0.002 (left panel) and η = 0.01 (right panel) are shown for a comparison. Under the effect of RAT-D, the slope of the extinction curve is steeper, resulting in lower R _V than the fixed value produced by the original dust (dashed–dotted black line). The value of R _V varies with increasing height z and tensile strength.