Panel A: 2D histogram of the vertical magnetic field (at log τ − 0.9) and the continuum intensity of AR 10923 observed on 14 November 2006 plotted on a logarithmic scale. Also shown are maps of the continuum intensity (panel B) and of B_z at log τ = −0.9 (panel C) of the spot. The field-of-view of these maps is the same as in Fig. 2. The contours in panels B and C indicate a continuum intensity of 50% of the quiet Sun level. In order to understand the shape of the histogram, we selected specific regions in the histogram and show examples of the corresponding features in the bottom two panels. Yellow triangles: outer parts of the umbra, red circles: spines, blue squares: transition from spines to filaments, green diamonds: dark central part of filaments, orange triangles: bright heads of filaments, and purple triangles: filaments tails exhibiting opposite polarity field. We reversed the sign of B_z to be positive in the umbra