The relationship between the total extinction, A(V) (measured in magnitudes), and the column density of neutral hydrogen atoms column, N H (usually measured in cm), shows how the gas and dust in the interstellar medium are related. From studies using ultraviolet spectroscopy of reddened stars and X-ray scattering halos in the Milky Way, Predehl and Schmitt found the relationship between N H and A(V) to be approximately: