  We use continuum simulations to study the impact of friction on the ordering
of defects in an active nematic. Even in a frictionless system, +1/2 defects
tend to align side-by-side and orient antiparallel reflecting their propensity
to form, and circulate with, flow vortices. Increasing friction enhances the
effectiveness of the defect-defect interactions, and defects form dynamically
evolving, large scale, positionally and orientationally-ordered structures
which can be explained as a competition between hexagonal packing, preferred by
the -1/2 defects, and rectangular packing preferred by the +1/2 defects.
