 In Figure 5, MC implies that X screens Y off from all of the other variables, and that W screens Z off from all of the other variables. This is most easily seen from MCScreening off. W also screens T off from all of the other variables, which is most easily seen from MCd-separation. MC does not imply that T screens Y off from Z (or indeed anything from anything). While Y and Z do have a common cause that screens them off (W), not all common causes screen them off (T does not have to), and not everything that screens them off is a common cause (X screens them off but is not a common cause).