The GMT-Master is a wristwatch model from Rolex . It has been produced since 1954.

The GMT-Master was introduced in 1954. It was the result of a request for proposals from Pan Am . The airline wanted a wristwatch for its pilots and flight crew that could display at least two time zones simultaneously. Rolex responded by designing a watch that could display a second time zone using a fourth hand.

The fourth hand is a distinctive feature of this watch. It's a second hour hand with a triangular tip. It rotates completely around the dial once every 24 hours, not every 12.

This second hour hand indicates the time not on the watch's regular dial, but on a bezel mounted around the dial. Accordingly, this bezel also features a 24-hour scale. The "regular" 12-hour hand on the GMT-Master II can be adjusted in one-hour increments without affecting the display of minutes, seconds, and the 24-hour hand.

The GMT-Master doesn't have this function (it has a classic quick-set date at the first crown position). Setting the second time zone is achieved by adjusting the bezel. Thus, the wearer of the GMT-Master II can quickly switch to a new, local time zone using this quick-set function of the "normal" hour hand without the watch stopping, while the 24-hour hand continues to display the home time.

The bezel can be rotated in both directions. By turning the bezel clockwise or counterclockwise, one can quickly detect a third time zone using the GMT-Master II's 24-hour hand. In its original version, the bezel was blue and red to indicate day and night. Watches in this coloration are called "Pepsi." ^{[ 1 ]}

Using the 24-hour hand, you can also determine exactly where north is. Align the regular hour hand with the sun, and the 24-hour hand will then point north. This only works if both hands indicate the exact local time and you are in the northern hemisphere.

Source: Wikipedia, AI