The spectral lines observed in penumbrae are asymmetrical, revealing the existence of spatially unresolved structures. We infer their physical properties by fitting Stokes spectra of Fe I 6301.5 Å and Fe I 6302.5 Å with model atmospheres having two magnetic components interleaved along the line-of-sight (LOS). Combining ten thousand fits, we set up a semi-empirical model sunspot, whose two components present very different magnetic-field inclinations. The major component, which contains most of the mass, is more vertical than the minor component, which is inclined below the horizontal plane throughout the penumbra. Magnetic field lines and mass flows are parallel, consequently both upflows and downflows are present everywhere in the penumbra. Major and minor components have very different velocities (several 102 m s-1 vs. 10 km s-1), but they transport the same mass per unit time. The similarity between the vertical mass flow and the magnetic flux of the two components suggests that field lines emerging as major component may return to the photosphere as minor component. If so, the observed magnetic field strength difference between components leads to a siphon flow whose magnitude and direction agree with the Evershed flow. Several tests support the internal consistency of the retrieved model sunspot, in particular, the magnetic field vector B does not violate the div B=0 condition. A detailed description of the techniques and results presented in this meeting is given by tet{s1 SA05}.