 The four boxes may be formally represented by a set \(W=\{w_{1},w_{2},w_{3},w_{4}\}\), typically called a set of possible worlds. Each world is further labeled with the atoms true at that world. They are labeled by a function V, the valuation. The valuation specifies which atoms are true at each world in the following way: Given an atom p, \(V(p)\) is the subset of worlds at which p is  true.[2]  That \(w_{1}\) is labeled with p and q thus means that \(w_{1}\in V(p)\) and \(w_{1}\in V(q)\). In the illustration, \(V(p)=\{w_{1},w_{2}\}\) and \(V(q)=\{w_{1},w_{3}\}\).