Why navies build ships in pairs

Pairs are useful when machinery, armament, or another technology is changing quickly: the second ship provides a check against bad luck, crew error, or a single operational loss. They can also serve as a cautious intermediate step between larger classes. This half-step allows a navy to evaluate one major change before compounding it with another. Such caution reflects the stakes of naval procurement, where a large design mistake can damage national security rather than merely waste money.