Left: three-component extinction models for the four M31 sightlines along with the average MW extinction for comparison. Each panel shows the model fit to the extinction curve, including the contribution of each component. The fraction of the available Si, Fe, and C (amorphous carbon and graphite) utilized is listed in the figure legend. Right: three-component extinction models for the same sightlines. Each panel contains the resulting mass distributions relative to the mass of hydrogen.