Directed and valued graph responding to these rules:

- The sum of the values (probabilities) associated with the edges coming from the same vertex gives 1,
- The probability of a path is the product of the probabilities associated with the edges that compose it,
- The value of an edge that goes from vertex S
_{1}to vertex S_{2 }is the conditional probability of the event S_{2 }knowing that the event S_{1}has already occurred.