 The relationships in the graph are often described using the language of genealogy. The variable X is a parent of Y just in case there is an arrow from X to Y. \(\bPA(Y)\) will denote the set of all parents of Y. In  Figure 1,  \(\bPA(Y) = \{T, W\}\). X is an ancestor of Y (and Y is a descendant of \(X)\) just in case there is a directed path from X to Y. However, it will be convenient to deviate slightly from the genealogical analogy and define “descendant” so that every variable is a descendant of itself. \(\bDE(X)\) denotes the set of all descendants of X. In Figure 1, \(\bDE(T) = \{T,X, Y, Z\}\).