Joint Placement: Spine

There's an amusing Simpsons gag I'm very fond of. I'm sure you know it -
Some local children are watching a member of a film crew paint a horse in cow-print. One of the children inquires,
    "Sir, why don't you just use real cows?"
    
    "Because cows don't look like cows in the movies - you gotta use horses",
comes the reply, matter-of-factly.
And for good measure, another of the children then asks,
    "What do you do if you want something that looks like a horse?"

    "Hm... usually we just tape a bunch of cats together."

I find myself thinking about that statement often. "Cows don't looks like cows in the movies"
You'd be amazed just how often that kind of thinking comes up when trying to make something that needs not necessarily to look real under scrutiny but to feel real in the moment.

Knowing how to properly place points of articulation in a rig is about knowing when to use a cow and when to use a horse painted to look like a cow. That is - when to mimic reality and when to spit in its face.
Another way of thinking about this is the difference between simulation and emulation.

The Spine

Spines are... interesting. Rarely would you bother giving a character rig the realistic curvature of an actual spine. It's usually either straight, or bending uniformly back.
The spine is pretty important and delicate. Anyone who's ever had back pain knows what a cruel mistress the spine can be. But in rigging, the spine is actually pretty forgiving.
But there are two things you should be aware of.

1) The spine is further inland of the back than you think it is.
People feel the bony ridges of the spine in the centre of their back and get the idea that the spine is placed more towards the back of a torso. And yeah, it is, but only slightly.
Look at a diagram of a human from the side and see that those bony ridges you're feeling are just the tip of the iceberg, and that the vertebrae reach pretty deep into the torso.

For a really cartoony character it's worth ignoring this question of placement entirely and just putting the spine straight through the middle of the torso.
For anything more realistic than that, further back is fine, but don't end up doing this - the results won't be pretty:

and 2) The bending isn't distributed evenly throughout the spine. Find some videos demonstrating the articulation and you'll see that most of the bending happens in the lower part of the spine, between the pelvis and rib cage.

This is why a lot of simple character rigs get away with two or sometimes even just one joint in the spine, and no division from the chest upward.

But don't therefore go thinking that the ribcage is completely rigid. It's made up of multiple interlocking ribs for a reason. It's so it can flex and change overall shape as you move and breathe.

In a lot of rigs you can get away with the spine around the chest being completely rigid, but in a more anatomically accurate rig keep in mind that the ribcage does bend in more strenuous poses, just not as much.

The move I like to have the usual spine flexing only effect the spine from the pelvis to the base of the ribcage, but also include an extra control to bend the chest if that's what the animator wants: