The GMT mechanism
A GMT watch adds a 24-hour rotating hand and a 24-hour scale (either on the bezel or the dial). The 24-hour hand makes one full rotation per day instead of twice. Set the 24-hour hand to UTC/GMT (or any reference zone), and you read your home time on the standard 12-hour dial plus a second zone via the 24-hour hand on the 24-hour scale. Modern traveler GMTs let you jump the local hour hand independently when you change time zones; caller GMTs jump only the 24-hour reference hand.
The world-time mechanism
Louis Cottier's 1937 patent uses a fixed 12-hour dial, a rotating 24-hour ring inside it, and a city ring graduated for all 24 reference cities (one per zone). Read your local zone on the 12-hour dial; read any other zone by looking at the 24-hour ring against the city. The mechanism shows all 24 zones at once; the trade-off is dial complexity and reduced readability of the local time.
When to choose which
GMT for daily wear: cleaner dial, easier to read, more practical for tracking one or two zones (e.g. office and family abroad). The Rolex GMT-Master II, Tudor Black Bay GMT, Omega Aqua Terra GMT are all volume options at CHF 4,000-15,000.
World time for prestige: more dial drama, every zone visible at once. Patek 5230, Vacheron Patrimony World Time, JLC Geophysic Universal Time. Retail CHF 25,000+. The mechanism is more complex, the dial is busier, the wear is more occasion-specific. See /styles/gmt/.
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