The market for a good with a positive consumption externality is typically illustrated with a graph showing two benefit curves. The demand curve represents the marginal private benefit (MPB). The marginal social benefit (MSB) curve, which includes both the private benefit and the marginal external benefit (MEB), lies above the demand curve. The supply curve represents both the marginal private cost (MPC) and the marginal social cost (MSC), assuming no production externality. The free market reaches equilibrium where the MPB curve intersects the MSC curve, resulting in underconsumption. The socially optimal equilibrium occurs where the MSB curve intersects the MSC curve. The triangular area between these two quantities, bounded by the MSB and MSC curves, represents the deadweight loss from underconsumption.