The two-country model (every paper)

Two separate supply-and-demand graphs, side by side — one per country — settling at a SINGLE trade price between their two original domestic prices.

Reading the graph

Each country has its own domestic price BEFORE trade — read where its own S and D cross.
The trade price (Pt) sits between the two original domestic prices, and is the SAME in both countries once trade opens.
The country whose original price was LOWER becomes the exporter — it now supplies more at the higher Pt than its own consumers demand.
The country whose original price was HIGHER becomes the importer — its own producers supply less than its consumers demand at the now-lower Pt.
X (the exporter's surplus) must equal M (the importer's shortfall) at Pt — that equality is what pins down where Pt sits.

A demand shift in ONE country moves the trade price in BOTH

When demand rises in the importing country (e.g. US demand for NZ grass-fed beef grows), the trade price itself rises — which then changes the exporting country's domestic quantity supplied too. Papers ask you to relabel BOTH graphs' new export/import quantities (X₁, M₁), not just the country where the shock happened.

What removing a tariff does to Pt and quantities

A tariff previously held the price away from the true two-country trade price — removing it lets Pt move to its natural level, raising BOTH the export and import quantities.

Practice for this standard covers the whole topic — flashcards and quiz.