  The symmetry of the alignment mechanism in systems of polar self-propelled
particles determines the possible macroscopic large-scale patterns that can
emerge. Here we compare polar and apolar alignment. These systems share some
common features like giant number fluctuations in the ordered phase and
self-segregation in the form of bands near the onset of orientational order.
Despite these similarities, there are essential differences like the symmetry
of the ordered phase and the stability of the bands.
