 The corresponding concept for undirected graphs is a forest , an undirected graph without cycles . <unk> an orientation for a forest produces a special kind of directed acyclic graph called a polytree . However there are many other kinds of directed acyclic graph that are not formed by <unk> the edges of an undirected acyclic graph . Moreover , every undirected graph has an acyclic orientation , an assignment of a direction for its edges that makes it into a directed acyclic graph . To emphasize that DAGs are not the same thing as directed versions of undirected acyclic graphs , some authors call them acyclic directed graphs or acyclic <unk> . 
