A break-open shotgun is a type of shotgun that loads shotgun shells by opening up the barrels, taking out the spent shells, placing a new shell in each chamber, then closing the barrels. Most are either single- or double-barrel. Triple- and quadruple-barrel (four barrel) shotguns also exist, but single- and double-barrel shotguns are easier to make and less likely to jam.
Most double-barrel shotguns are made with their barrels touching side-by-side. However, there are also shotguns where one barrel is on top of the other. Over-and-under shotguns are easier to aim than side-by-side shotguns because you do not need to aim a little to the left or right.
Since the metal cartridge was invented in 1845, most shotguns were break-open shotguns until the repeating shotgun was invented. However, break-open shotguns are still popular with civilians because they are easier to use and less likely to jam than repeating shotguns. Nonetheless, they hold fewer shells than repeating shotguns.