Did you know Alexander is credited with inventing the wristwatch?
Alexander had trouble coordinating the maneuvers of his commanders spread out over large areas, so he had his tutor, Aristotle, come up with a scientific solution to the problem.
It was known that cloth, dipped in certain chemicals, would slowly change color over a few days as the chemical dried out. He had some cloth so treated, and cut into strips which each commander wore as an arm band, along with written orders to respond to the color changes. Something like, "when the cloth turns green, advance on the left flank, all other units stand fast and prepare for counter-attack by the enemy."