Double-barreled shotguns come in two basic configurations: side-by-side (S×S) and over-and-under ("over/under", O/U, etc.), indicating horizontal and vertical arrangement of barrels. The original double-barreled guns were nearly all S×S designs, which was a more practical design for muzzleloaders. Early cartridge-firing shotguns also used the S×S action, because they kept the exposed hammers of the earlier muzzleloading shotguns from which they evolved. When hammerless designs started to become common, the O/U design was introduced, and most modern sporting doubles are O/U designs.