Triangular Trade is a good go-to policy for those who have plenty of Trade Route Trade Routes, be they domestic or international, for this applies to both.
When slotted, this policy gives +4 Gold and +1 Faith to all Trade Route Trade Routes, which can make the money really start to roll in. This is a great policy for civilizations dependent on trade and those short on money. The +1 Faith can help out religious civs too, either when buying military units (with Theocracy or the Grand Master's Chapel) or Missionaries and Apostles to spread their religion.
Triangular trade routes are those that connect three locations, each with a different commodity desired in one of the other two, in a profitable cycle of merchant voyages. As the colonial empires grew, shipping captains sought to make their trips across the Atlantic Ocean ever more efficient. One such was the British colonial route: salt fish and lumber from New England to British colonies in the West Indies, sugar and molasses from the islands to Britain, and manufactured goods from Britain to New England. But the most profitable was surely the transatlantic slave trade operating from the 1500s to 1800s; engaged in by various nations, slaves from West Africa to the Caribbean and American colonies, cash crops and raw materials from there to Europe, and manufactured goods to Africa. And repeat …