Which hot dog bun style is common in most of the United States?

The side-loading bun is the hot-dog bun style common in most of the United States.

A side-loading bun is split lengthwise along one side, creating a pocket into which the sausage and toppings are placed. It is the familiar supermarket and concession-stand style found across much of the country. The bun’s shape makes it easy to manufacture, package, transport, and serve with standard toppings such as mustard, relish, onions, and ketchup.

The main regional contrast is the New England–style, or top-loading, bun. Instead of being cut along the side, it is sliced from the top and usually has flat, standing sides. That design is especially associated with New England lobster rolls, clam rolls, and hot dogs.

“Brioche” and “pretzel” describe types of bread or flavor profiles, not the dominant American cut. A Chicago-style hot dog may use a poppy-seed bun, but it is still generally a side-loading bun. The distinction concerns where the bun opens, not whether it is toasted or what toppings it carries.