In common usage, "culture" means "the customary beliefs, social forms, and material traits of a ... social group" (m-w.com). In EU2, culture is simpler: it's basically just a number. In EU2, there are lots of cultures, but two cultures are either the same, or they are different. There is no such thing as "similar".
Countries have one primary culture, but they may also have secondary culture(s). You can see the state culture(s) on the country screen. The first culture listed is the primary state culture. For example, the Ottoman Empire has as its primary culture Turkish, but it also has Arab, Greek, and Slavonic culture.
State cultures are established by the scenario file for all countries which exist when the game is started or loaded. Upon new country creation (via revolt, freeing a vassal, etc.), the new country always gets a single culture, usually that of its capital province.
Every land province has exactly one "base" culture. If the province has a trading post, and it has not had a colony on it that was then destroyed, you can see its base culture on the province screen and the send missionary screen. You cannot see the base culture of a province if it has ever had a settlement on it, or if it is empty. The base culture is fixed by the scenario and cannot change.
Every land province may have a settlement (a colony, colonial city, or full city); and if it does, that settlement has exactly one culture. Settlement culture is independent of the base provincial culture. This culture is shown on the province screen.
Cultural Change in Settlements
Events can change any settlement's culture to any culture. For example, in the scripted event where the Ottomans can move their capital to Thrace, if they make that choice they get cultural conversions of several provinces nearby to Turkish culture (as well as some nice religious conversions). Scripted events changing culture are rare; no random event changes culture.
When a colony is created, it gets the primary state culture of the country creating it. After that, settlement culture usually does not change much. However, there are two actions that the country owning a settlement may do, that may change the settlement's culture. These always change settlement culture to the country's primary state culture. The actions are:
- successfully expanding a colony, as long as the result of the operation is still a colony.
- successful religious conversion of any settlement with pagan religion also converts the province's culture.
There is one other way that culture can change. In non-European provinces that contain colonial cities (1000-4999 pop), whenever nationalism ends, the colonial city's culture will change to the owner's primary state culture. This effect only works if the settlement culture is different from the base provincial culture. Note that nationalism usually lasts 30 years; you can see it via the increase in revolt risk. However, when a country gains ownership of one of its core provinces, nationalism ends immediately (and the culture changes if other conditions apply). Similarly in the case of a colonial city defection, there is no nationalism, and therefore there is immediate culture change.
Again, there are two very substantial limits on this effect: the colonial city must be non-European, and it must not match culture to the base provincial culture (which you can't see). Base provincial culture usually matches the settlement culture for settled provinces as they are in the Grand Campaign scenario. (A few exceptions exist, including the Canary Islands.) Thus, for example, China cannot easily expand into the Changatai Khanate and convert its many small cities into Han culture, because those cities are Mongol cities on provinces with Mongol base culture.
Penalties for Cultural Diversity
Culture has a number of effects. For a particular country, a wrong-culture city/colony/province is one that does not match any of its state cultures. A wrong-culture settlement:
- is harder and costlier to colonize. The modifier is +8% for colonizing a settlement or province with your state culture. (When colonizing unsettled provinces, the provencial base culture is used; otherwise the settlement culture.)
- is much harder and costlier to religiously convert.
There are many more effects in cities. A wrong-culture city:
- has a -30% on province tax income.
- has reduced recruitment potential (number of armies you can build there). This is actually a side effect of the province tax income reduction.
- has a +1 modifier to the revolt risk, +2 when at war.
- has amplified revolt risk from war exhaustion.
- supplies reduced manpower.