Meek's rules

R1: input

Start with the partially directed graph.

R1: nodes 0, 1, 2; orient 1 -> 2

A directed edge enters a node with an undirected edge. Orient the undirected edge away from that node when the outer nodes are not adjacent, avoiding a new collider.

R1: nodes 1, 2, 3; orient 2 -> 3

A directed edge enters a node with an undirected edge. Orient the undirected edge away from that node when the outer nodes are not adjacent, avoiding a new collider.

R2: input

Start with the partially directed graph.

R2: nodes 0, 1, 2; orient 0 -> 2

A two-edge directed path connects the ends of an undirected edge. Orient that edge along the path, avoiding a directed cycle.

R3: input

Start with the partially directed graph.

R3: nodes 0, 1, 2, 3; orient 0 -> 3

Two nonadjacent parents point into a shared child. A node with undirected edges to both parents and the child must point into the child.