Histogram of relative orientation (Soler et al. 2013) throughout the simulations High_B (top) and Low_B (bottom). The graphs highlight the transition from parallel alignments in the low-density regime to more perpendicular ones as the density increases driven by the strength of B, which sets the threshold of this transition. From the last column we can evince how the expansion of the H II region promotes parallel alignments between the density structures and the magnetic field.