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. Trade zones give a bonus to city income (not castle or temple income) and trade post income.
For trade zone purposes, control of a sea province is determined by which family owns the most trade posts adjoining that sea province. If there is a tie, the sea zone is considered neutral (no family owns it). Trade posts owned by the same Patrician family in provinces that share a land border are part of the same trade zone, but a patrician family's trade zone can only span sea zones if the family controls the sea zone(s) being spanned.
Trade zones scale super-linearly, such that it's 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 and then clicking on a republic while holding down the Cntrl key.
Trade Zone Bonus
Cities and trade posts within a trade zone receive a percentage bonus to their tax income depending on the trade value (TV) of the trade zone. Increasing trade value provides diminishing returns as the bonus will approach 100% as the Trade Value approaches infinity. Note that this bonus is halved for cities.
Trade Zone Bonus = (TV * 100) / (TV + 200)
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