There are two types of crown law: Crown authority, and for Catholics, investiture. These two laws together have a large effect upon your realm and I'll go into what they do, what advantages they bring, and what disadvantages they bring.
As always, there's a summary at the end.
The Basics
The two crown laws decide what authority the king or emperor of a realm has. Crown authority represents the centralization of powers to the monarch, while investiture decides who controls who becomes bishops; the Pope or the King.
Crown laws only apply within the de jure realm they're set to. Thus if you conquer territory in another de jure kingdom, their crown laws will apply to those holdings. The aspects of crown law that apply specifically to vassals will be based on where the vassals have their capital. It is entirely possible for a vassal to be getting the opinion modifier for high crown authority for example, while most of his holdings are in a kingdom with lower crown authority.
Crown laws can be put up for vote at any time as long as you don't have a regent, you're not in a civil war, and you haven't changed a crown law with that character.
Investiture
Investiture laws is a small set of laws that only apply to Catholics; no other religions will have it. Investiture law decides if the Pope appoints bishops, or if their liege does. This means that if you have Papal Investiture you will be unable to affect who your bishops are. Papal Investiture does however, reduce the piety cost for papal favors by 50%, so if you enjoy frequent excomunications, divorces, and crusades, stick with the Pope. Free Investiture is a bit more complicated.
If you've got Free Investiture you'll be able to appoint the successor to any bishopric that is your direct vassal, and every subject within your de jure kingdom will be able to do the same. This makes vassal bishops considerably happier with their lieges, as they are grateful for having been appointment, and they therefore get +25 opinion with whoever their liege is. Any vassal you have that himself has bishops as vassals will also be happy with you, as he now has control over his bishops. You'll thus get +10 opinion with most of your vassals.
However, the Pope will be quite unhappy with this state of affairs. You will get -30 opinion with the Pope making you much more vulnerable to excommunication. In addition, every time a new king gets the throne the Pope will request, generally within a few years, that the investiture laws are changed to Papal Investiture, which will give you the option to comply (which will raise the moral authority of the Church by 2%), refuse (reduces MA by 2%), bribe the Pope (costs 200 gold), or if you have a Learning skill of 16+, refuse on theological grounds (gives you 1 piety).
This means that over time, Free Investiture does have the potential to be a moderate money drain. However, as long as your realm is moderately large, the gold needed for a single Papal bribe is unlikely to be more than a couple year's income at most. The largest disadvantage to Free Investiture is the much increased risk of excommunication, and as long as you have political enemies and a lack of traits the Church likes, you can easily end up excommunicated, giving every Catholic in the world a casus belli against you, and reducing all your vassals' opinion of you by 30.
Crown Authority
Crown Authority decides how much power the ruler has, and affects several aspects of your realm.
The first, and perhaps most important aspect of crown authority regards the size of the levies that your vassals provide you with. This starts at 0% at Autonomous Vassals, and increases by 20% per level, to a max of 80% at Absolute Crown Authority. This overrides both Opinion and Levy Law as long as it is higher. Higher crown authority thus means that you're able to call on larger levies in times where your vassals dislike you, for example: right after succession.
Further, each step of crown authority gives the king privileges.
At Limited Crown Authority the king gets the ability to revoke titles and the ability to appoint generals. This level of crown authority is extremely important, as this will let you strip a single title from any vassal that rebels, thus allowing you to redistribute power in a way that makes the realm more stable. Second, you will no longer risk being stuck with horrible commanders in an army stack, as you'll be able to appoint someone more suited to the role. At times, this can win you fights, and potentially wars.
At Medium Crown Authority the crown gets another two important privileges; free revocation of titles from infidels, and no fighting between vassals. By being able to revoke titles from infidels and heretics you'll no longer have to deal with vassals having large Infidel/Heretic penalties, and you'll be able to redistribute their titles as you see fit. No infighting between vassals means that vassals will no longer be able to go to war against each other except when rebelling against their liege, thus you'll almost always have their levies available when you need them. It will also prevent vassals from getting powerful via intra-realm warfare.
Upon passing High Crown Authority titles can no longer be inherited by anyone outside the realm, so you will no longer lose parts of your realm due to inheritance. Foreign nobles will no longer be able to snag your lands from right under your nose, and inheritance gets more predictable.
Absolute Crown Authority gives one final privilege: vassals can no longer go to war (except to rebel) at all. This means that your vassals will no longer be able to grow powerful by taking land from outside the realm, and makes them thus more manageable. The drawback is of course that you now cannot grow that way either; you will personally have to start all conquests.
Increasing crown authority will not only grant privileges, but also bring some pretty severe penalties. First and foremost are the opinion penalties. Whenever you increase crown authority, you immediately incur a -30 opinion penalty with all of your vassals. This immediate penalty persists for 10 years, however you will also recieve a permanent opinion penalty, the severity of which depends upon your current level of authority. At Autonomous Vassals you'll get +5 opinion with all your noble vassals, but this rapidly goes down. At Limited, you get -5 opinion; at Medium you get -10; at High you get -20; at Absolute you get -30. This makes vassals less willing to provide tax and levies, and more likely to rebel.
Increasing crown authority also increases the risk of rebellion directly. At Autonomous Vassals all vassals will have a -20% chance of rebelling, but this increases by 10% each level, ending at +20% at Absolute Crown Authority. Beyond this and the opinion penalty however, there are very few drawbacks. In total you get -35 opinion from going to max authority, and +40% rebellion risk (+17.5 to 35 from the opinion hit, for a total of 57.5 to 75%).
Due to how severe the rebellion and opinion penalties are, I would not recommend going beyond Medium Crown Authority. At this point you've got the most important privileges without incurring too large penalties. Unless you often lose land to inheritance there is no real point to going to High Crown Authority as it is seldom worth the opinion and rebellion hit.
Briefly, if you're a vassal, the ideal realm crown authority would be limited authority. This allows you to appoint generals and revoke titles and, more importantly, still allows you to war against other vassals in your realm for more land.
Crown authority is also needed to set some of the succession laws. You need Medium authority for Seniority, and High for Primogeniture. You can lower the crown authority afterwards without losing the succession law.
Summary
Investiture law decides if you or the Pope appoints bishops. Free Investiture makes your vassals happy, and Papal Investiture makes the Pope happy.
Crown Authority decides how much power the king has, but incurs major opinion and rebellion penalties.
My tips are as follows:
- Unless you often get excommunicated, use Free Investiture
- Medium Crown Authority generally gives the best of both worlds
- Go to High Crown Authority if you're often losing land to inheritance
- If you want to switch to Seniority succession, you'll need Medium Crown Authority
- If you want to switch to Primogeniture succession, you'll need High Crown Authority
- Limited Crown Authority is an absolute minimum; you need the ability to revoke titles
Guide written by Meneth