For marriage specific outcomes between spouses see Marriage Guide.
As Crusader Kings II is based around your dynasty, getting the most out of marriage is essential. Marriage is the primary way to expand your dynasty, and to ensure its continuation. In addition, marriage can be used to forge alliances.
I'll go into how the marriage mechanic works, and how you can use it to your benefit.
As always there's a summary at the end.
The Basics
Anyone above the age of 16 can get married. Anyone below 16 can get betrothed. There is no max age at which one can get married, but women become completely infertile at 45.
Marriage brings several benefits, the most obvious of which is child birth. In addition, you also get half of your spouse's stats added to your own for the purpose of state statistics, and for determining demesne size. You'll also get prestige if the house your spouse is from is a prestigious one, and you'll get or lose prestige depending on the rank of your spouse, 20 prestige per step above or below you. So if for example you as an emperor marry a king's daughter, you'll lose 20 prestige while she'll gain 20.
Further there are many events, both good and bad, tied to being married. The traits of your children will also be affected by the traits of your spouse, and you can assign your spouse as spymaster as the position is limited to males, the ruler's mother, and the ruler's spouse.
Children
As long as the female in a marriage is under 45 and neither character has the Celibate trait, she can potentially get pregnant. The chance of pregnancy is affected by several traits, like Chaste and Lustful. It is also possible for her to get pregnant through events, for example if your male character becomes rather angry with the death of his first-born son.
Couples tend to have two children on average, but traits increasing fertility can make a couple produce far more than this. My personal record is 9 children, and I've heard of people having as many as 15 (my personal record is 21, all female). So as long as you don't have any fertility reducing traits, you should be able to get enough children to continue your dynasty.
The stats and traits of a child will be affected by the stats and traits of both parents. As such if you want a good heir it is wise to marry someone with good stats and traits. Do note that a mother will have very good relations with her own children, but not the children from a previous pairing. As such if you remarry and have more children, you risk your new wife assassinating your original children to pave the way for her own.
Statistics
Half of your spouse's stats will be added to your own for most purposes. This includes all the state statistics and your demesne limit.
As your spouse affects your demesne limit, a high Stewardship spouse means you'll be able to hold several more holdings, 1 for every 8 points (s)he has in Stewardship. For reference, you can hold 1 extra holding for every 4 points you have in Stewardship.
If your spouse has higher numbers in any stat, you'll be likely to get occasional events where you'll get the opportunity to ask your spouse for help. This usually gives you a positive temporary modifier to some aspect of your realm, and a 20% chance of getting 1 point in that skill. However, there's also an 80% chance to get the Content trait, which means you cannot get the extremely useful Ambitious trait (+2 to every stat)
Marrying someone with high stats can therefore be very useful, and can make a big difference in how well you're able to control your realm.
Alliances
Marrying someone or their son or daughter, or marrying your son or daughter to them or their son of daughter, will give you an alliance. However, unless they join your dynasty either through you/your son marrying patrilinealy or you(female)/your daughter marrying matrilinealy, this alliance will not persist for future generations. As such, many marriages will only confer a temporary alliance, not a lasting one.
If you want a lasting alliance, you'll have to ensure the children are of your own dynasty. The alliance will then persist forever unless a later generation leaves your dynasty.
By marrying your children to other powerful lieges, you'll be able to call these characters into your own wars and thus have them assist you. This is however a two-edged sword as they can do the exact same with you, and if you decline the call to arms you'll get a loss to prestige and take an opinion hit.
Overall Strategy
When you're marrying someone, or marrying your primary heir to someone, your first priority should be good stats and traits. Avoid anyone with big losses to fertility, and anyone above 30, as they'll be much less likely to produce the children you need. Especially seek positive genetic traits, full list below:
- Attractive: Gives 30 opinion with the opposite sex, +1 diplomacy. Especially useful if a female heir gets it.
- Genius: +5 to all stats, +5 opinion with all vassals. EXTREMELY good trait
- Quick: +3 to all stats. Very good trait.
- Strong: +1 Diplomacy, +2 Martial, +2 Health, +10% Fertility, +10 opinion with the opposite sex, +10 opinion with all vassals. Extremely good trait.
On the other hand, avoid the negative genetic traits as these could end up plaguing your heirs for generations. Full list below:
- Clubfooted: -10 opinion with the opposite sex, -1 martial
- Harelip: -10 opinion with the opposite sex, -1 diplomacy
- Hunchback: -30 opinion with the opposite sex, -5 opinion with all vassals, -1 martial
- Lisp: -5 opinion with the opposite sex, -1 diplomacy
- Stutter: -1 diplomacy
- Ugly: -20 opinion with the opposite sex, -1 diplomacy
- Dwarf: -30 opinion with the opposite sex, -1 martial
- Slow: -3 to all stats
- Imbecile: -8 to all stats, -10 opinion with all vassals. Extremely bad trait.
- Inbred: -5 to all stats, -30% fertility, -1.5 health, -20 opinion with all vassals. Even worse than Imbecile.
- Weak: -1 health, -5% fertility, -1 martial, -10 opinion with the opposite sex, -10 opinion with all vassals.
When you're marrying off children who won't be your heir, these concerns are far from as important. You should still try to avoid the worst of the genetic traits however, as you don't ever want to be stuck with them.
Your primary priority when marrying off children is instead to expand your dynasty and to gain alliances. If you get the chance, marry your daughters matrilinealy. Try to avoid marrying your sons matrilinealy, as the children being a part of your dynasty is of great benefit, as then the alliance will persist through the generations.
If you get the opportunity to marry in a way that preserves your lineage, not the other dynasty, try to marry the ruler or his/her primary heir, but only if they don't have children already. A second batch of children of your dynasty is far from as useful as primary heirs of your dynasty.
Further, try to avoid marrying people that are already in your dynasty, as then you risk the negative genetic traits. While the risk is very low once you get a few steps away (E.G., second cousins), it generally isn't worth it. So unless you have a very good reason to do so, never marry anyone in your own dynasty. You don't want to end up like Charles II of Spain.
Contributed by Meneth