Lesson IX: Sacra Romanum Imperium Nationis Germanicae
This lesson was originally going to be about building trees. Well you know what? Screw building trees!
Actually, no, there is a reason. I wrote it, posted it, and then the forums killed it. I was so annoyed I didn't exactly want to write it again. So instead, here we go!
Before we get to the main attraction, let's check out our progress in terms of technology. Conveniently, there is a new feature in 5.1 called the "technology mapmode". It's a mapmode that contrasts our technological advancement with that of the rest of the world in a visual form. Previously, you had to go to the ledger and check each individual category. Now it's much easier to check how much more advanced you are than your terrible mouth breathing neighbors.
Eastern Europe is red, Muslimland is all green and stuff, and most of the west is orange, though there's some exceptions. Some of the larger but still rich countries have some kind of.. grorange going on. It's sort of green and sort of orange and.. well, I can't really put my finger on it. However, if you look in the Lowlands, there's a little bright green speck lit up like the second sun, a beacon of civilization and savior of the educated ideal.
The great Red Lion, Holland!
(Yes, I seriously just was that melodramatic.)
Okay, enough of that. Just another example of how INCREDIBLY POWERFUL TRADE IS. (Figure that out yet?

)
This is our main deal. (Ignore the fact that it's Austria, it was the first HRE screen picture I had lying around. Or, uh.. digitizing around. Wait, that's not the right word. Um... Bah. Screw it)
The big shield at the top is the flag of the Imperial homeland, followed by the name of the emperor and his home country. Below that we have seven mid-sized flags. Collectively, these seven are the Electors of the Holy Roman Empire, who determine the next emperor when the current one dies. Historically, they could be impeached, but that's not really an option. So if you just want a short-term position, feel free to just win the election then stop caring what the rest of the nation thinks. (This is a good way to stop the Empire from unifying, since you can kill the authority of the Imperial office)
Following that, we have a bunch of tiny flags. These are the members of the HRE, who gain the benefits of Imperial protection (when the emperor feels like it), relative peace (except when one nation gets too powerful), and free cores upon inheritance (assuming you don't get PU'd first). Yeah, actually, when you put it that way it sounds like a terrible thing.
However, members have it pretty easy when it comes to being elected (non-members get an electoral penalty) and thus it has uses that way. In fact, the three things I listed above are pretty good, under normal circumstances. However, let's talk about the benefits of being Emperor.
First off, you get huge forcelimits and manpower increases. Stability is so cheap you almost never have to invest in it. You get free CBs on tons of nations, many of which lead to free core provinces. Finally, you can unify the empire under a single banner using the reform system. Not bad, eh? (There's also other benefits, but these are the main ones)
So, how do you go about getting elected? Well, you need the majority electoral vote upon the death of the current emperor. How you get voted for depends on the elector themselves.
You see, if the elector is a kingdom, you can get a Royal Marriage for a good +20 points. However, if the elector is a theocracy or archbishopric (or, later on, a republic) that goes out the window. Instead, you'll need to use other means. For all electors, high relations are a must. It's a good idea to get all electors to 200, if you plan on becoming emperor. Vassals will most likely vote for, unless you're a terrible, terrible candidate (Vassal/union gives you +250 electoral points with the respective elector)
High infamy will lower your electoral points. You need to be of the same religion as the elector, unless they are your vassal and you are simply a heretic (no one will ever vote for infidels). If you are a current emperor vying for a second term, authority gives you electoral points (one point for one authority) which can be useful.
It's a good idea to secure as many electors as possible, because the current emperor will generally steal your electors back at a relatively high rate. If you have a lot, that's a higher security that you will win upon the death of the current emperor.
So there you go! The Holy Roman Empire in a nutshell. It can be a bit confusing at first, but if you play Austria or Bohemia, you'll get the hang of it pretty quick. (Bohemia is my personal favorite for a first game, since they start as emperor AND they're also an elector)