Generally, inhibitor arcs connect a place to a transition such as their function is to prevent the transition from firing whether the connected places have the tokens imply. Everyone knows that the transitions fire only while the connecting input places has tokens represent in it. Though, situations may arise to follow a reverse route, such to prevent the transition from firing while token is represented. Inhibitor arcs are defined in a same fashion like the normal arcs along with an exception that they prevent or inhibit the transition from firing instead of permitting the transition to fire while connected place has tokens. An easy case to present this case is presented in the figure. In the following figure (g) the arc originating from place p1 and terminating on transition t1 is named as inhibitor arc. This is shown by a line with a filled or hollow circle at its ending.
Figure (g): Inhibitor Arcs in a Petri Net