When deconfounding variables $X$ and $Y$, a back-door path is any path from $X$ to $Y$ that starts with an arrow pointing into $X$. (Notice that if there is a collider on the back-door path, the path is already blocked by the collider.) The path $A \leftarrow B \leftarrow C \rightarrow D \rightarrow E$ is a back-door path from $A$ to $E$. Forks are also back-door paths.