3D coronal magnetic field structures before the onset of the X2.2 and X9.3 flares, which are reconstructed by the NLFFF extrapolation. All the images depict AR as seen from the southeast (i.e., from the lower left in the images of Figure 2). Panels (a) and (b) correspond to the magnetic field structure before onset of the X2.2 flare (calculated from SDO/HMI SHARP on 08:36 UT on 2017 September 6), whereas panels (c) and (d) correspond to that before the onset of the X9.3 flare (calculated from SDO/HMI SHARP on 11:00 UT on 2017 September 6). The different-colored tubes indicate magnetic field lines with different magnetic connectivities. Panels (a) and (c) depict the side view of the magnetic fields with a bottom image of the radial magnetic field (B_z obtained from SDO/HMI SHARP). Panels (b) and (d) depict the same view with the background of AIA 1600 Å images at the time of the initial flare ribbons at 08:59:27 and 11:55:27 UT on 2017 September 6, which are the same images as Figures 2(e) and (h), respectively.