Armor thickness means little without belt depth and displacement

Why mention the 18-inch belt without specifying how far it extends above and below the waterline?

Because the effective belt position changes with real loading and displacement; the thickest armor is a limited central citadel, not a uniform full-length belt.

A headline armor thickness does not reveal how much of the belt is actually useful. A very thick but shallow belt can be submerged when added topweight changes the ship's real displacement and waterline. Drawings and nominal design figures therefore have to be compared with photographs of the vessel afloat. For Benbow and similar ships, the heaviest armor is concentrated around the central citadel rather than extending uniformly along the entire hull. In practice, this resembles an all-or-nothing distribution.