It starts out as the long section of meat between the shoulder blade on one side and the ribs and backbone on the other.
(The muscles on the outer side of the shoulder blade are where we get the other major chuck subprimal, the chuck shoulder clod.)
The butcher will first bone out the shoulder blade, then remove the ribs and backbone.
Finally, the meat from the lower rib area is removed and made into ground chuck.
What's left, after trimming away fat and squaring it off, is a large (20-ish pounds), a boneless hunk of meat called the chuck roll.
Because it's tough, fatty, and/or gristly, half of it ends up as ground beef, stew meat, stir fry meat and so on.
However, it's possible to get some good quality steaks and roasts from it. Doing so requires separating it into two common cuts known as the chuck eye roll and the chuck under the blade.
Chuck Eye Roll
The chuck eye roll (or simply "chuck eye") is an interesting piece of meat, because, at the rib end, it contains a few inches of the same tender muscle that gives us ribeye steaks.
However, it's also surrounded by connective tissue, fat and a few other muscles that aren't so tender.
A typical approach is to make those first few inches into steaks, known as chuck eye steaks or sometimes, amusingly, called Delmonico steaks.
(Chuck eye steaks merit their own discussion, and I've discussed them at length. If you're interested, you can read more about chuck eye steaks.)
The middle section of the chuck eye can be sliced into thick strips and sold as country style ribs, which are flavorful and excellent for braising.
Finally, the tough meat from the neck end of the chuck eye is often used for stew meat or ground chuck.
Or, it might be sold as a chuck eye roast. But obviously, beware of any roast that could also be stew meat.
Also see: How to Roast Meat
For that matter, sometimes the whole chuck eye is simply cut in half and sold as two large roasts.
A butcher might split them lengthwise (giving the opportunity to trim out fat and possibly fill them with some kind of stuffing), then tie them back together or wrap them in butcher's netting. My advice, though, is to avoid this thing.
Think about it: This exact same piece of meat is sliced into strips and sold as country style chuck ribs, which need to be braised. Cutting it in half and wrapping it up like a roast doesn't change the fact that it's a tough piece of meat that needs slow, moist-heat cooking.
The chuck under blade consists of three muscles, the rhomboids, the serratis ventralis and the splenius.
The rhomboidius is extremely tough, so the first step is removing it for ground beef or stew meat. The splenius can then be detached from the serratis ventralis.
The main thing with this muscle is that it has a lot of connective tissue on the outside, which needs to be completely trimmed away.
You can prepare a Sierra steak much like a flank steak: marinate it, grill it over high heat and slice it across the grain.
The serratis ventralis (also know as the chuck edge roast or chuck flap) is a long, relatively tender, well-marbled muscle that can be made into steaks. But again, it needs to be denuded of all exterior connective tissue.
If you've ever seen (or eaten) something called a Denver steak, this is where it came from.
Also see: What is the Best Steak?
The serratis ventralis can be cut in half along a natural seam where the muscle fibers change directions. This is important, because steaks from this muscle need to be sliced against the grain, or they'll be chewy.
One technique is to separate the rear section and slice it against the grain into Denver steaks.
The front half can then be cut into steaks or made into stew meat, kabobs or stir-fry meat — not necessarily because it's less tender, but because its pointy shape makes it difficult to fashion into steaks.
To save time, the whole muscle is frequently portioned into Denver steaks (without separating the two sections first). Alas, steaks cut this way will not be cut uniformly against the grain, so your jaws are going to get a workout.
Also see: Why You Need to Have a Great Butcher