The two consumers of a public good have marginal benefit curves MB₁ and MB₂. At the efficient point, both consumers consume Q*_Pub of the public good, where their total marginal benefit curve (MB_T) intersects the marginal cost curve (MC). If the two consumers privately bought the good, each would consume less than the optimal level at quantities Q₁ and Q₂, where MB₁ and MB₂ each intersect MC.