Mainland Portugal occupies the Atlantic side of Iberia. The Tagus estuary at Lisbon makes a useful anchor: a narrow entrance opens into a much broader body of water inland. Farther south, the Sado estuary and the turn toward the Algarve provide another coastal sequence to trace.
Checking the location
Compare this coast with Galicia to the north, where drowned valleys create deep inlets. Inland farming patterns and dry summer colours overlap with western Spain and cannot establish the border. This regional image covers the mainland; Madeira and the Azores are separate Atlantic island groups and need their own comparison.