 An arrow from Y to Z in a DAG represents that Y is a direct cause of Z. Roughly, this means that the value of Y makes some causal difference for the value of Z, and that Y influences Z through some process that is not mediated by any other variable in \(\bV\). Directness is relative to a variable set. We will call the system of direct causal relations represented in a DAG such as  Figure 3  the causal structure on the variable set \(\bV\).