Illustration of the effects of clumping and porosity in velocity space. Each sketch shows the star (yellow) and its clumpy wind (different shades of grey). In the left figure we indicate different parameters that describe the wind structure. In the middle and right sketches we illustrate how the effective transparency of spectral lines in the wind depends on volume filling fraction as well as on the velocity span of the clumps. Depicted are photons of different frequencies (v_ı in red and v₂ in blue, with v₁ < v₂) that are intercepted by a strong absorption line if they are Doppler shifted with the right velocity. The corresponding resonance zones lie at 0.5v_∞ and 0.6v_∞ (shaded red and blue, respectively).