 We will adopt the convention that both DAGs and ADMGs represent the presence and absence of both direct causal relationships and latent common causes. For example the DAG in  Figure 1  represents that W is a direct cause of Y, that X is not a direct cause of Y, and that there are no latent common causes. The absence of double-headed arrows from Figure 1 does not show merely that we have chosen not to include latent common causes in our representation; it shows that there are no latent common causes.