 The last of these, of course, is the Common Cause Structure characterized by Reichenbach. (However, Reichenbach also postulated that intermediate causes would screen off distal causes from their effects, as indicated in the first two diagrams.) On the other hand, suppose the relations of probabilistic (in)dependence are exactly the opposite. That is: X and Z are probabilistically independent, but dependent conditional on Y. In this case, there is only one causal structure that implies just this set of probabilistic independence relations: