For a long while, wristwatches were thought to be fashionable for ladies only. The guys stuck with their pocket watches—until the First World War. WWI was the first time airplanes were used in combat. Sometimes several airplanes were used for a coordinated attack, which meant they had to arrive at the target at the same moment. When you’re hundreds of feet in the air, working a joystick and firing a machine gun, you don’t have enough hands to also pull out your pocket watch to see if you’re on time (why didn’t they put a clock in the plane’s dashboard? I don’t know).
Likewise, if infantry soldiers in the trenches were ordered to open fire on the enemy simultaneously at a pre-planned time, a timepiece on your wrist is a whole lot more convenient than one in your pocket to count down the seconds while you’re holding a machine gun and a shovel.
So designers began designing manly-looking wristwatches for the guys.
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