Armor design was a question of weight, area, and metallurgy

Armor schemes expanded from relatively narrow belts toward broader belts, thicker decks, and sometimes multiple protected decks as guns and expected battle ranges changed. Designers ordered protection by weight per unit area rather than by nominal inches alone because the ship had to carry the total mass across every protected surface. Improvements in high-tensile steel and alloys containing nickel, chromium, and other additives could change the protection delivered by a plate of a given thickness and weight. That made metallurgy, layout, armor trials, and shell performance inseparable parts of the design problem. John Jellicoe faced problems with the quality of armor and shell testing, while Reginald Bacon's knowledge of gun manufacture suited the technical demands of his office.