Sketch of the two coronae approach. A hot (kT ∼ 100 keV) and optically thin (τ ∼ 1) corona (in red) is present in the inner parts of the accretion flow close to the black hole. This produces the hard X-ray emission through Comptonization of the soft photons coming from the outer part of the flow. The outer accretion flow is a vertically structured accretion disk (a zoom is shown in the figure) with cold and optically thick matter in the deeper layers (in black), characterized by a temperature T_bb and threaded by a vertical radiative flux F_disk, while the upper layers (in orange) are composed of a warm (kT ∼ 1 keV) and optically thick (τ_cor ∼ 10–20) corona possessing a source of internal heating power Q(τ). The warm corona is also illuminated from above by an illumination flux F_Illum produced by the hot corona. This sketch is partly inspired from Różańska et al. (2015).