Importance of the wind/photosphere transition region on the emergent spectrum of Cyg OB2 #12. Left: influence of the transition velocity (in units of the sound speed, v_sound) on the spectral typing diagnostic lines He i 4471 and Mg ii 4481 Å. Right: influence on the Fe iii/Fe ii ionization equilibrium diagnostic lines. The Fe ii 5170 Å line is severely affected by the transition velocity while the Fe iii lines remain roughly unaltered.