In the presence of a negative externality there is a positive-valued Marginal External Cost (MEC). This implies that the Marginal Social Cost (MSC) is greater than the Marginal Private Cost (MPC). Also, in the presence of a positive externality there is a positive-valued Marginal External Benefit (MEB). This implies that the Marginal Social Benefit (MSB) is greater than the Marginal Private Benefit (MPB)