 A second type of graph that we will consider is an acyclic directed mixed graph (ADMG). An ADMG, will contain double-headed arrows, as well as single-headed arrows. A double-headed arrow represents a latent common cause. A latent common cause of variables X and Y is a common cause that is not included in the variable set \(\bV\). For example, suppose that X and Y share a common cause L (Figure 4(a)). An ADMG on the variable set \(\bV = \{X, Y\}\) will look like Figure 4(b).