Abreast of the Special Treatment Steel (STS) outer hull plating, the "South Dakota"s internal armor belt consists of thick Class A Krupp cemented (K.C.) armor mounted on thick STS plate, and was inclined 19° from the vertical. This was equal to 17.3 inches (440 mm) of vertical belt armor at 19,000 yards. The belt armor extended to the triple bottom with a Class B homogeneous Krupp-type lower belt and tapered from its maximum thickness of down to at the lowest portion at the triple bottom. This feature was chosen to protect against penetration of heavy-caliber gun projectiles that managed to hit the ship below the waterline. The ends of the armored citadel are protected by thick traverse bulkheads. The horizontal deck protection is made of three layers: a STS weather deck (also called "bomb deck"), a combined Class B and STS second deck, and a STS splinter deck over the machinery spaces. Over the magazines, the splinter deck is replaced with a STS third deck. The immune zone of the side armor against the 16-inch guns used by the "South Dakota"s themselves was smaller due to the introduction of the 2,700 lbs Mark 8 Super Heavy shell; against such a shell, the armor was effective only at ranges between .