Once upon a time, beef chuck came from the meat purveyor as a big, bone-in primal cut called a square-cut beef chuck. It's a complex jumble of meat and connective tissue (i.e. gristle), along with bones from the ribs, shoulder blade, and neck.
In the old days, about half of the beef chuck would be made into roasts (or steaks), and the rest ended up either in the grinder or sold as stew meat.
And that's still done today. There will always be a market for these classic beef chuck roasts. And beef chuck also happens to be the best meat for making burgers, because it has just the right fat content.
Nevertheless, these days butchers (and chefs) have a lot more options for how to order and fabricate beef chuck.
By the way, in meat cutting, the word "fabricate" means to cut a large primal cut into smaller subprimal cuts or to cut subprimes into individual steaks, roasts, chops, and so on.
One of the most common ways of fabricating a beef chuck is by separating it into two major boneless subprimal cuts, the chuck roll, and the chuck shoulder clod.
The Chuck Roll
The chuck roll is a large (around 20 pounds) boneless subprimal made up of the long section of meat from between the ribs and the backbone.
A skilled butcher can remove the ribs and backbone in one piece. This piece of meat can then be divided in half, with the section that overlays the ribs used for ground beef. What's left–after being trimmed and squared up–is called the chuck roll.
The chuck roll actually has some fairly tender muscles in it, including a few inches of the longissimus dorsi, which is the same exact muscle from which we get ribeye steaks. There are also quite a few tough muscles in the chuck roll, and just like the shoulder clod, the chuck roll can be fabricated further into even smaller sections.
One of the most common techniques is to separate the top section, called the chuck eye (which contains the tender longissimus dorsi muscle), from the bottom section, which is known as the chuck under the blade.
The Chuck Shoulder Clod
The shoulder clod is basically the big lump of muscle on the top side of the animal, which forms its outer shoulder bulge. Like the chuck roll, the shoulder clod is also usually around 20 pounds.
Separating the shoulder clod from the beef chuck requires cutting around and extracting the upper arm bone (called the humerus) and then carefully cutting the muscle away from the shoulder blade bone.
A shoulder clod is a group of five muscles that can be separated and fabricated into steaks and roasts. The advantage of separating these muscles is that it allows the connective tissue between them to be removed as well. All this connective tissue is one reason beef chuck can be so chewy if it's not cooked properly.
That's the good news. The bad news is that most of the muscles from the shoulder clod are still tough, even with the connective tissue removed.
Finally, there's another muscle on the outside of the shoulder blade called the supraspinatus, which sits just forward of the shoulder clod. It's commonly known as the chuck tender.