Directed acyclic graphs

visual depiction of a partially ordered set

directed graph with no directed cycles

an ordering of the vertices of the directed graph for which each edge is directed from earlier to later in the ordering

thumb|A polytree In mathematics, and more specifically in graph theory, a polytree (also called directed tree, oriented tree or singly connected network) is a directed acyclic graph whose underlying undirected graph is a tree. In other words, a polytree is formed by assigning an orientation to each edge of a connected and acyclic undirected graph.