Perseus molecular cloud H I and CO velocities (LSR) from HI4PI Collaboration (2016) and Dame et al. (2001), respectively. The red circles show velocities pointing away from us. This figure shows our first approach for determining the v_{CO - H I} direction, as described in Sect. 4.1. The size of the circles shows the magnitude of the velocities. The background gray-scale image shows the extinction map in A_v magnitude as obtained by Kainulainen et al. (2009). Left panel: circles show the H I velocities. Right panel: circles show the CO velocities.