The general shape of the ultraviolet through near-infrared (0.125 to 3.5 μm) extinction curve (plotting extinction in magnitude against wavelength, often inverted) looking from our vantage point at other objects in the Milky Way, is fairly well characterized by the stand-alone parameter of relative visibility (of such visible light) R(V) (which is different along different lines of sight), but there are known deviations from this characterization. Extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features.