 A path in a directed graph is a non-repeating sequence of arrows that have endpoints in common. For example, in  Figure 1  there is a path from X to Z, which we can write as \(X \leftarrow T \rightarrow Y \rightarrow Z\). A directed path is a path in which all the arrows point in the same direction; for example, there is a directed path \(S \rightarrow T \rightarrow Y \rightarrow Z\). A directed graph is acyclic, and hence a DAG, if there is no directed path from a variable to itself. Such a directed path is called a cycle. The graph in Figure 1 contains no cycles, and hence is a DAG.