  We present observations and magnetic field modeling of the large polar crown
prominence that erupted on 2010 December 6. Combination of SDO/AIA and
STEREO$\_$Behind/EUVI allows us to see the fine structures of this prominence
both at the limb and on the disk. We focus on the structures and dynamics of
this prominence before the eruption. This prominence contains two parts: an
active region part containing mainly horizontal threads, and a quiet Sun part
containing mainly vertical threads. On the northern side of the prominence
channel, both AIA and EUVI observe bright features which appear to be the lower
legs of loops that go above then join in the filament. Filament materials are
observed to frequently eject horizontally from the active region part to the
quiet Sun part. This ejection results in the formation of a dense-column
structure (concentration of dark vertical threads) near the border between the
active region and the quiet Sun. Using the flux rope insertion method, we
create non-linear force-free field models based on SDO/HMI line-of-sight
magnetograms. A key feature of these models is that the flux rope has
connections with the surroundings photosphere, so its axial flux varies along
the filament path. The height and location of the dips of field lines in our
models roughly replicate those of the observed prominence. Comparison between
model and observations suggests that the bright features on the northern side
of the channel are the lower legs of the field lines that turn into the flux
rope. We suggest that plasma may be injected into the prominence along these
field lines. Although the models fit the observations quiet well, there are
also some interesting differences. For example, the models do not reproduce the
observed vertical threads and cannot explain the formation of the dense-column
structure.
