A trade zone is a continuous set of provinces whose trade posts are owned by the same character. Trade zones give a bonus to city income and trade post income. Trade zones do not scale linearly, but it's still usually better to have a single 6-province trade zone than two 3-province trade zones.
Patrician family trade zones can be viewed by clicking on the trade zone map mode (G) and then clicking on a republic while holding down the Ctrl key.
Contiguity[edit | edit source]
For trade zone purposes, control of a sea province is determined by which family owns the most trade posts pointing into that sea province. If there is a tie, the sea zone is considered neutral: no family owns it, and no trade posts can be connected through it. If the sea zone is controlled by a family, all trade posts in adjoining land provinces are connected, regardless of port locations.
Trade value[edit | edit source]
The value of the trade zone is based on the income and holdings in the provinces with trade posts. Trade zone value also receives a large bonus from being connected to the republic's capital. Some buildings increase trade value as well.
Income bonus[edit | edit source]
Cities and trade posts within a trade zone receive a percentage bonus to their income depending on the trade value (TV) of the trade zone. Note that this bonus is halved for cities.
Trade Zone Bonus = (TV * 100) / (TV + 200)
Disease spread[edit | edit source]
With Reaper's Due DLC, trade zones can help spread epidemics. Wealthy rulers should consider fortifying their trade zone provinces with upgraded hospitals.
Technology spread[edit | edit source]
Technology spreads unidirectionally from trade posts to the merchant republic capital.