Comparison of an ATLAS 12 (blue) and TLUSTY (black lines) atmospheric structures for a model with T_eff = 20 000 K, log g = 2.50, and ξ =10 km s⁻¹. The upper and lower panel depict the run of temperature and the electron density, respectively, as a function of the Rosseland optical depth τ_R. The line formation regions of Hγ and two lines of He I are indicated in the upper panel. See the text for a discussion.