 We only need to represent missing common causes in this way when they are closest common causes. That is, a graph on \(\bV\) should contain a double-headed arrow between X and Y when there is a variable L that is omitted from \(\bV\), such that if L were added to \(\bV\) it would be a direct cause of X and Y. Double-headed arrows do not give rise to “genealogical” relationships: in  Figure 4(b),  X is not a parent, ancestor, or descendant of Y.