  We consider the physical origin of the hemispheric pattern of filament
chirality on the Sun. Our 3D simulations of the coronal field evolution over a
period of 6 months, based on photospheric magnetic measurements, were
previously shown to be highly successful at reproducing observed filament
chiralities. In this paper we identify and describe the physical mechanisms
responsible for this success. The key mechanisms are found to be (1)
differential rotation of north-south polarity inversion lines, (2) the shape of
bipolar active regions, and (3) evolution of skew over a period of many days.
As on the real Sun, the hemispheric pattern in our simulations holds in a
statistical sense. Exceptions arise naturally for filaments in certain
locations relative to bipolar active regions, or from interactions between a
number of active regions.
