AM versus column-density. The horizontal line is the zero line and marks the boundary between the parallel case (AM > 0) and the perpendicular case (AM < 0). The vertical line marks the position of TN, which is around A_V=10. The uncertainties of the AM and the columndensity for each data point are shown as red error bars (see Appendix F for further details). The trend is clear here: filaments and the B field tend to be parallel below TN, while they become perpendicular beyond TN.