Latitude could be observed directly; longitude required comparing local time with the time at a reference meridian. A timekeeper that held a known rate at sea therefore became a navigational instrument of national importance.
The eighteenth-century work associated with Harrison's timekeepers, and the later refinement of the detent escapement by makers including Earnshaw and Arnold, produced a design that could be made repeatably rather than as a single masterpiece.
The standardised chronometer that emerged has a recognisable form: a fusee-driven movement with a spring detent escapement, a large compensation balance, gimbal mounting in a brass bowl, and a wooden box with winding record.
Rate, not absolute accuracy, was the working requirement. A chronometer was issued with a rate certificate and its daily gain or loss applied as a correction, which is why original certificates and rate books matter so much to surviving examples.
Chronometer practice fed back into pocket watches. Detent escapements, compensation balances and positional adjustment all reached the wider trade from this line of work.