 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 directed from X to Y. \(\PA(Y)\) will denote the set of all parents of Y. In  Figure 3,  \(\PA(Y) = \{T, W\}\). 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 also a descendant of itself. \(\DE(X)\) denotes the set of all descendants of X. In  Figure 3  \(\DE(T) = \{T, X, Y, Z\}\).