The areas 3 + 4 represent the gain in producer surplus in Germany.

Price stability then leads to greater joint welfare (the sum of consumer and producer surplus.

Also, consumer surplus and producer surplus are the same under such an import quota and a tariff.

Producers are better off because the producer surplus (yellow region) is made larger.

The reservation price is used to help calculate the consumer surplus or the producer surplus with reference to the equilibrium price.

Likewise, in the supply-demand diagram, producer surplus is the area below the equilibrium price but above the supply curve.

In technical terms, it is the increase of consumer surplus plus producer surplus from lower tariffs or otherwise liberalizing trade.

The benefit to producers of the price support is equal to the gain in producer surplus (represented in blue).

When examining tax incidence, it is the lost consumer and producer surplus that is important.

So the profit is equal to the sum of cosumer surplus and producer surplus.