The battleship USS Alabama (BB-60) under construction at the Norfolk Navy Yard on 3 July 1942. At the time of this photo, Alabama was nearing completion, with commissioning on 16 August 1942. The photo particularly highlights her forward turrets. The number 1 turret still lacks its faceplate; what is more noteworthy is that the backing plate is missing. During construction, the turret interiors of United States Battleships were sheathed with plates of Special Treatment Steel (STS). These STS plates served as backing to the actual armor plates that were then attached to them. Unlike structural steel, STS had protective qualities and actually contributed to the overall armor of the turret. STS steel was similar to homogeneous Class B armor used by the US Navy (As opposed to face-hardened Class A armor). In addition to its protective qualities, STS steel was also elastic and resisted tearing. This made it very effective as a backing plate, as it helped contain spalling, shrapnel, and shell fragments. When listing the armor of the South Dakota class battleships, some publications list the 18" (457mm) of armor on the turret faceplate only. However, this ignores the protection afforded by the backing plate underneath. The faceplates of the South Dakota class were actually 20.5" (521mm) thick due to the STS steel. Factoring in the STS plate, the true armor thickness of the South Dakota class turrets would look like this: Turret Front: 18" (457mm) Class B armor + 2.5" (64mm) STS Turret Sides: 9.5" (241mm) Class A armor + .75" (19mm) STS Turret Rear: 12"(305mm) Class A armor + .75" (19mm) STS Turret Roof: 7.25" (184mm) Class A armor + .75" (19mm) STS Now back to Alabama In the photo, Alabama is missing her STS backing plate on the turret front. Like her sisters, Alabama utilized STS backing plates on the turret sides, rear, and roof. However, her turret faceplates were completely different in that they did not use STS backing. Alabama experimented with single plates of armor for the turret faceplates. Rather than combine Class B and STS, Alabama simply utilized a thicker slab of Class B armor, using a single plate that was just shy of 19" (483mm) in thickness. Presumably, this was done because a single plate of homogeneous Class B should be of equal effectiveness to two plates of homogeneous Class B and STS, even if the two plates had a combined thickness that was slightly greater. The equality in protection would be benefited by simpler construction and a slight savings in weight. Alabama was the only member of her class to experiment with this style of faceplates. However, it seems that the potential advantages never materialized as the system was not repeated in subsequent designs. The Iowa class returned to mounting Class B armor over STS for the front of its turrets. The Montana class also used Class B and STS backing, though with far greater thicknesses.