Projected gas density (ρ) around two galaxies from our sample, G23 (upper panel) and G11 (lower panel). These two panels illustrate each of the two successive phases leading to the formation of gas-versus-stars counter-rotating disks. In the upper panel, the central galaxy (G23) has a counterclockwise rotation, as specified by the dashed circular arrow, and is accreting some gas from a companion galaxy through tidal stripping. In this specific case, the orbit of the satellite galaxy is indirect (or retrograde) as illustrated by the orientation of the two arrows. Then, the retrograde accretion of gas may lead afterward to the coexistence of two disks of gas that rotate in opposite sense, as shown in the lower panel by the spectacular case of G11. The accreted gas lies in the outer parts of the galaxy while the pre-existing one is concentrated in the inner parts. The process ends when the latter is totally replaced by the former.