 \(\mathcal{G}\) is a set of ordered pairs of variables in \(\mathbf{V}\). If \((X, Y) \in \mathcal{G}\), we represent this graphically by drawing an arrow from X to Y, and we say that X is a parent of Y. If there are arrows from \(X_{1}\) to \(X_{2}\), \(X_{2}\) to \(X_{3}\),… and \(X_{n-1}\) to \(X_{n}\), then there is a directed path from \(X_{1}\) to \(X_{n}\). In this case, we say that \(X_{n}\) is a descendant of \(X_{1}\). \(\mathcal{G}\) is acyclic if there is no directed path from any variable to itself.  Figure 9  shows a DAG on the variable set \(\{U, V, W, X, Y, Z\}\). In this DAG, there is a directed path from U to Z, and Z is a descendant of U. \(\PA(X)\) is the set of all parents of X and \(\ND(X)\) is the set of all variables in \(\mathbf{V}\) other than X and its descendants. In  Figure 9,  \(\PA(W) = \{U, V\}\) and \(\ND(W) = \{U, V, X\}\).