Figure 1.

An example of how tokens move among places. In a Petri net, quantities of tokens are assigned to places. In (a), three tokens are assigned to place p_A and zero tokens are assigned to place p_B. The two places are connected by a transition, t₁. The arcs in and out of t₁ indicate the direction in which tokens move. When t₁ fires, it moves some number of tokens from p_A and puts them in p_B. In (b), transition t₁ has fired and moved two tokens from p_A to p_B.