World time
A rotating twenty-four-hour ring read against a fixed ring of city names, showing every time zone at once.
Time zones were standardised internationally from 1884, and within a generation the telegraph, the liner and the aeroplane made it ordinary for a person in one zone to need the hour in another. The conversion was done in the head, which is exactly where it goes wrong: booking a call, filing an order, making a connection.
Louis Cottier, working alone in Carouge, arrived at the solution around 1931. A ring marked with twenty-four hours is geared to the hour hand so that it makes one turn a day, and it is read against a fixed ring carrying the name of one city in each zone. The hour anywhere is read straight off, and every zone is legible at the same moment. Cottier refined it in 1953 by adding a second crown to index the city ring, which is the form the complication still takes.
The cost is legibility and precision. The dial carries two dense rings of small text around a conventional set of hands, and the mechanism assumes whole-hour offsets, so the half-hour and quarter-hour zones used by India, Iran, Nepal and parts of Australia cannot be shown correctly. Daylight saving, which moves in different months in each hemisphere, is not modelled at all.