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How does a GMT watch work?

sorry for the stupid question... Is it as simple as just using the GMT hand as a second hour hand for a different here? "...11 NASA tests, 90 flights in space, 118 missions, 6 moon landings, ONE WATCH."
Answer:
There are several types of GMT watches. They all have at least 3 hands. A minute hand (makes one full revolution in 1 hour), a hour hand (makes one full revolution in 12 hours) and a 24 hour hand (makes one full revolution in 24 hours)

1) The main hour hand can be independently moved forward or backwards in 1 hour increments (clicks) with the crown or pushers while the 24 hour hand is advanced only by setting the watch (setting the minute hand will rotate the 24 hour hand)

2) The 24 hour hand can be independently moved forward or backwards in 1 hour increments (clicks) with the crown or pushers while the hour hand is advanced only by setting the watch (setting the minute hand will rotate the hour hand)

3) The 24 hour hand can be independently set to any position with the crown, there are no distinct clicks relative to the minute hand.

The first kind of watch is what you'd like to ahve if you were a true traveller where you can quickly adjust the hour hand to local time while the 24hr hand would retain the home time at all times.

The second type of watch is ideal if you want to monitor a 2nd time-zone with your watch while staying home (like I live in the US but I have relatives in Europe, I would set the 24 hour hand to indicate the time in Europe)

The third kind is a cheap imitation of a GMT watch, it is not a true GMT movement, cheap Chineese "GMT" movements have this "feature", the brand called "Alpha" has such a movement.

I hope that answered your question.
Answer:
There 2 more types that I'm aware of:

1) The 24-Hour hand is not independent of the (regular) hour hand; the bezel is used to keep track of the second time zone (Rolex GMT-Master)

2) There is a subdial for the second time zone; the hour hand can be adjusted independent of the second time zone subdial like the first type Greg Takacs mentioned. (Maurice Lacroix Pontos Grand Guichet GMT)
Answer:
The 24-Hour hand is not independent of the (regular) hour hand; the bezel is used to keep track of the second time zone (Rolex GMT-Master)

Correct, the GMT Master keeps track of the second time zone with the unidirectional bezel, which is a tricky way of making a GMT watch, I would not call it a true GMT in current modern time. On the GMT Master II the GMT hand can be adjusted separately allowing a total of 3 time zones to be set (main hour hand, 24 hour hand, bezel position).

The Omega Speedmaster day-date has a 24 hour hand on the seconds subdial (at the 9 o'clock) but it cannot be individually adjusted and since there is no rotating sub-bezel around it, it cannot be used to show the time in a second time zone.
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