Here's a diagram of where the two come from:
Sometimes the best "center cut" slices are removed from half hams during processing. If the label on a half ham reads "shank end half" or "butt end half", then it includes these meaty center slices. If it says "shank end portion" or "butt end portion", then the center slices have been removed.
When choosing a whole, bone-in ham, authors Bruce Aidells and Denis Kelly of The Complete Meat Cookbook write, "A short, plump shape with a stubby rather than an elongated shank is the best choice."
There's a lot of conflicting information about which type of half ham is the best one to buy. Cook's Illustrated writes, "You're better off getting a shank end rather than the butt end. This is because the butt end has less desirable meat, as it contains a lot of membrane, fat, and gristle. Its bone configuration can also make carving tricky." However, some butchers recommend a butt end ham, saying that it's meatier and has better flavor.
It's not clear to me that there's a big difference, and often when you visit the supermarket you just end up having to pick from what's available based on price and the size of ham you're looking for.
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