Trade involves the movement of surplus trade goods from one province to another, producing 
commerce income for both the importer and the exporter. It is one of the two main ways that a country produces income and allows a country to profit from surplus production, potentially allowing countries with valuable and exotic trade goods to become much wealthier than their population base would suggest. An overview of all trade routes in the country as well as various automatic controls can be viewed in the 
Trade Overview screen.
Trade Routes
- 参见：Trade goods
A 
trade route is the movement of a trade good from one province, the exporter to another, the importer. One of the main effects of a trade route is to effectively move trade goods from one province to another, meaning that the importing province will get the bonuses from the imported trade good as though it was produced there, and the exported will not get the bonus from the exported trade good even though it was produced there. This is particularly useful for accumulating surpluses of trade goods in the 
capital province, as well as obtaining food trade goods if the population of a province outstrips its food supply.
Trade routes are also important for the commerce income that they give to both the importing and exporting provinces. The base commerce value of a trade route is determined by the base trade value of the trade good, varying between 
0.20 gold and 0.50 gold with valuable and exotic goods like 
gemstones or 
incense worth much more than common trade goods such as 
grain and 
wood. The base trade value is then multiplied by a certain factor based on the the type of the trade route, and whether the province is the importer or the exporter:
- ×1 for the exporter in a foreign trade route
- ×0.35 for the importer in a foreign trade route
- ×0.20 for both the importer and the exporter in a domestic trade route
The commerce income for the importing province is then modified by the province and country 
import value modifiers, and similarly by the province and country 
export value modifiers for the exporting province. As it is commerce income that is generated, trade route income is also modified by the local and national commerce income modifiers. Note that all countries begin with a base +20% export value modifier, so all exports are in fact worth more than they would at first appear.
Therefore, foreign trade routes are always more valuable than domestic trade routes and should always be preferred if possible, compensating for how they are more difficult to create (and keep). Exporters will also in general earn more gold from trade than importers, in exchange for losing the effect of the trade good. Generally speaking exporting is almost always better than keeping the trade good for its bonus, unless a capital surplus would be lost or the province needs the trade good for its 
food supply.
In order to create a trade route, the potential exporter must have a surplus of that good (i.e. produces at least 2 of that trade good), the potential importer must have at least 1 unused import route, and there must be trade access between the two provinces. Provinces belonging to the same country (i.e. domestic trade routes) always have trade access, but creating a trade route between provinces belonging to different countries (foreign trade routes) requires that the exporter be within 
diplomatic range of the importer (note that the inverse does not necessarily have to apply) and that the two countries are not at war. For foreign trade routes, the exporting country must also approve the trade, which they will generally do as long as 
relations are not poor and doing the trade will not result in the loss of a capital surplus bonus.
Creating a new import route also costs a base of 5 gold for the importer, modified by the 
Create Trade Route Cost modifier, given in particular by 
inventions and Mercantile stance.
Once a trade route is established, it will remain in place as long as the importing province has enough trade routes, the exporting province has enough of the trade good to fulfill all trades, both provinces remain under the same owner as when the trade route was established, and the two countries (if relevant) are not 
at war. If any of the conditions are not met, trade routes will be automatically cancelled until they are fulfilled again (particularly, this means that all trade routes will be cancelled if at war, which means that going to war with a major trading partner can result in considerable economic disruption).
As a passive modifier, every active import and export route also gives 
+2.50% pop promotion speed to every territory in the province.
Import routes
Every province has a maximum number of import routes that they are allowed to have. Once a province reaches its maximum, it will no longer be able to create more import routes, and if the number of import routes is higher than the maximum then the most recent trade routes created will be cancelled until the number of import routes is at the cap again. Note that there is no limit to how many exports a province can have, other than the amount of surplus trade goods it produces.
The base number of import routes in for each territory is determined by the sum of the 
Local base trade routes modifiers, modified by the Local base trade routes modifier; this is then summed up across all the territories in the province and then rounded down to get the baseline maximum number of import routes for the province. Local import routes are produced only by 
noble and 
citizen pops, who produce +0.15 Local import routes and +0.03 Local import routes per pop, respectively. Local base trade routes is determined by infrastructure - +10% Local base trade routes for each 
port in the province (applying to all territories, not just the territory the port is in), +5% Local base trade routes for every 
road connecting the territory to another territory, and +2.5% Local base trade routes for every 
marketplace in the territory, available only to 
cities or 
metropolises.
This baseline maximum can then be modified at a province level by the Local import routes province modifier, for all provinces in the country by the Province import routes country modifier, and for the capital province specifically by the 
Capital import routes modifier. Apart from as a bonus from various events, decisions, and missions, the 
Entice Business Investments province investment also gives +1 Local import routes, making it in general the fastest and easiest way to increase the number of trade routes a province has as long as the country has enough 
political power; the metropolis territory rank also gives +1 Local import routes. The Province import routes modifier is given only by the 
Institute Tariff Exceptions national idea, which gives +1 Province import routes. Capital import routes, though has many possible static sources, listed below:
| Type | Modifier | Capital import routes
|
|---|---|---|
| Country rank |  Regional Power
|
+1 |
 Major Power
|
+2 | |
 Great Power
|
+2 | |
| Inventions | Logistics Bureau (Civic Tech 1)
|
+1 |
| River Barges (Civic Tech 4)
|
+1 | |
| River Outposts (Civic Tech 6)
|
+1 | |
| Maritime Patrols (Civic Tech 10)
|
+1 | |
| Encourage Exploration (Civic Tech 14)
|
+1 | |
| Trade Ports (Civic Tech 8)
|
+1 | |
| Government | Plutocratic Monarchy (Monarchy)
|
+1 |
| Plutocratic Republic (Republic)
|
+1 | |
| Economic policies | Trading Permits (Commerce policy)
|
+2 |
| Deities |  Al-Quam (Arabic deity)
|
|
| Caleb (Jewish deity)
| ||
| National ideas |  Patronized Trading Posts (Civic Tech 11)
|
+3 |
| Heritages |  Minoan Heritage (Praesos)
|
+1 |
 Epikrateia Heritage (Epikrateia)
|
+1 | |
| Bosporan Heritage (cultural) | +1 |
Unused import routes in a province give +0.06 commerce income, which is not modified by import value but is still affected by commerce income modifiers. This means that increasing the amount of trade routes a province has is always beneficial even if no imports can actually be made, although importing actual trade goods will almost always produce more gold even before considering the additional benefits of having more trade goods in a province.
Commerce income
 |
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Trade routes create commerce income on both sides of the trade, with the bulk of the income generally going to the exporter. Commerce income is calculated on a per-province basis as the sum of all trade route income (including import routes, export routes, and income from unused trade routes), modified by the Province commerce province modifier and the National commerce income country modifier, the sum of which across all provinces a country determines the country's total commerce income. The value of each individual trade route is determined by the trade value of the trade good involved, whether the trade route is foreign or domestic and if the province is the importer or the exporter, and the effects of either import value and export value modifiers. Note that commerce income modifiers stack additively with each other, but multiplicatively with import value and export value modifiers as they modify the income from trade routes directly.
Important Commerce Income modifiers include:
- +2% per ruler
finesse (or Consort/co-Consul if higher) - +2.5% by each level of
Civic Advances - +5% with a capital surplus of
Dates - +2% for each
camels trade good in the province - +5% with the State Harbor Fees invention (requires Civic Advances 3)
- +10% with the Mandated Record Keeping invention (requires Civic Advances 20)
- -25% for regions with a
governor, plus an additional -0.125% per governor 
corruption - +2.5% per governor finesse plus a base of +2.5% with the
Encourage Trade governor policy - +2% per governor finesse plus a base of +2% with the
Bleed Them Dry governor policy - +1% per governor finesse plus a base of +1% with the
Acquisition of Wealth governor policy - +20% with the
Complex Tariffs National idea - +15% for various
pantheon deities - +12% per
100% omen power for various omens - +10% for Plutocratic Monarchies and Republics
- +15% with the Non-Interference Edict Monarchy law
- +20% with the Barter Economy Statutes Tribal law
- +25% when using the
Mercantile Stance - +20% for
City States - +5% for each
colony
参考资料