Culture is an abstract representation of the ethnicity and traditions of the peasantry, and whom characters identify with.
Cultures are divided into culture groups, and cultures are considered closer to others within their group than to cultures outside it. Culture mismatch can hurt both on the provincial level and inter-character relations. Penalties for different cultures within the same group are half those for cultures in different groups.
Culture determines certain diplomatic relations, as characters will get an opinion penalty towards anyone not of their culture. This penalty is even larger if they're not of the same culture group, but is reduced by the Cultural Flexibility Technology.
In addition to affecting character relations, culture also unlocks a series of buildings, making cultures a bit more diverse. Culture also affects some events, and certain cultures have their realm named after their dynasty, not their title.
In Crusader Kings II all cultures are created equal, there are no advantages inherent to any one culture beyond superficial things like names and portraits with the exception of a limited number of events that depend upon or are influenced by culture.
 The Provincial Level
Every county in the game has a culture representing the major culture there. At game start most realms will be relatively homogenous, with few realms containing more than one culture, and even fewer containing more than one culture group. However as a realm expands it will inevitably encounter other cultures and culture groups, which will give various penalties, both permanent and temporary.
The permanent penalty (unless culture changes) is a 1% revolt risk for being a different culture than the liege, and 2% if it in a different group. In addition upon conquering a region it will get a temporary penalty if it is a different culture. If it is in the same culture group it will get the following penalties, which are all doubled if it is of a different culture group:
As you can see, this means that a newly conquered area will take quite a while before it is of any real value, and even after the temporary penalties disappear you'll still have to deal with the revolt risk, which also reduces tax by 1% per 1% revolt risk. Also, as mentioned in the installment on religion, a county being of its liege's culture makes it considerably easier to convert the province.
 The Character Level
However as mentioned earlier, culture is not limited to counties alone. Every character in the game identifies with a culture, and this affects their relations with other characters in the same realm. Being of a different culture will reduce vassals' opinion of you by 10, and by 20 if you're of a different culture group. This penalty is modified by the cultural flexibility in the character's capital county. Second, being of the same culture reduces character revolt risk directly by 15%. Combined with the lack of an opinion penalty makes same culture vassals much more loyal, and homogenous realms much more stable than others.
 Spread of Culture - Character Level
Every character will be born with the culture of his father in a patrilineal marriage, or his mother in a matrilineal marriage. With very few exceptions, the only way to change this is via the child's guardian. If the guardian of a child is of a different culture there's a decent chance the child will change to the guardian's culture. The mean times to happen are as follows:
- Diligent and gregarious guardian - 30 months
- Diligent or gregarious guardian - 60 months
- No relevant traits guardian - 120 months
- Diligent + shy or gregarious + slothful guardian - 120 months
- Shy or slothful guardian - 240 months
- Shy and slothful guardian - 480 months
As you can see, a culture change happens about 50% of the time if the guardian has no traits modifying the chance, as a child generally has a guardian for 10 years. If the guardian is diligent and/or gregarious it becomes far more likely, as the normal distribution is then centered at 2.5 or 5 years, while on the other hand if the guardian is shy and/or slothful it becomes quite unlikely. So if you want to change the culture of your child, you should assign a guardian of the desired culture that has the diligent and/or gregarious trait.
 Spread of Culture - Provincial Level
Every county has a culture from the start of the campaign, but this can be changed over time. The event for a province changing culture works like this:
The event is called on a character-basis, not a county basis. Thus, the chance is not per county, but per character. For the event to trigger, the character has to have a county in his demesne not of his culture, and it must border at least one other province of the character's culture that is owned by a character (which could be the very same character) of the ruler's culture. Thus it is near impossible to spread any culture to islands, as they cannot possibly border a county of your culture unless it was present at game start. While there is an event that lets culture spread to culturally unconnected provinces, this event can only ever fire if the province is of a different religion, and as a province changes religion much faster than culture, this event will almost never fire. The British isles for example are thus for all practical purposes limited to (in the default campaign start)) Saxon, English, Welsh, Scottish, Irish, and Norwegian, while most other islands have only a single potential culture. Now, the event itself depends primarily upon a single factor: the ruler's stewardship, and the mean time to happen is as follows:
- 0-1 stewardship - 6075 months (506.25 years)
- 2 – 4050 months (337.5 years)
- 3 - 2700 (225 years)
- 4 - 1800 (150 years)
- 5-9 - 1200 (100 years)
- 10 - 900 (75 years)
- 11 - 675 (56.25 years)
- 12 - 506.25 (42.1875 years)
- 13+ - 380 (31.67 years)
The MTTH is also multiplied by 0.4 if the culture is Mongol and it is before the year 1350. As you can see, as long as a character has high stewardship he can convert counties to his culture rather quickly.
First, if you want to change your child's culture, assign him a guardian of that culture with the diligent and/or the gregarious trait. He'll be very likely to change culture. Conversely, if you want to keep your culture pure, be careful who you assign as his guardian.
Second, assign high-stewardship characters of your culture as counts on the edge of your culture. They'll rapidly change the culture to their own, getting rid of the revolt risk.
Third, try to have a single county per character. As the culture change event fires on a character level, a single character owning more than one county makes it less likely to happen on a per-county level.
Fourth, try to keep your vassals of your culture, as you'll get lower revolt risk and avoid the foreigner opinion penalty.
Finally, prioritize conquering same-culture provinces over provinces of a different culture. Prioritize same culture-group over different culture groups. You'll get less severe penalties that way.
By keeping your realm culturally homogenous you'll have an easier time controlling it. My tips are as follows:
- If you want to culture-change, appoint a Diligent/Gregarious guardian of a different culture
- Assign high-stewardship characters of your culture as counts in different culture provinces bordering your provinces
- Try to keep a single county per character
- Try to keep your vassals of your culture
- When conquering, prioritize same culture, over same culture group, over different culture group
 List of Cultures