Vertical axis: formation-depth of the average line-wing radiation according to the Milne-Eddington model; horizontal axis: continuum optical depth where the average Planck function is equal to the average line-wing radiation. Red dots are line cords 15 to 25 at μ = 0.9; green dots are line cords 15 to 25 at μ = 0.8, blue dots are line cords 15 to 25 at μ = 0.7. Dashed lines show the first bisector.