A semaphore is a system for sending messages using flags, lights, or mechanical arms in different positions. Each position represents a letter, number, or signal. Before radios and telephones, semaphore was one of the fastest ways to communicate over long distances.

The most famous type used two handheld flags. A person would hold the flags in different positions: arms straight out, one up and one down, both diagonal, with each position standing for a different letter. Ships at sea used semaphore flags to talk to each other when they were too far apart to shout but close enough to see. Scouts and sailors still learn semaphore today, though mostly for tradition and emergency backup.

On land, tall towers with movable arms called semaphore towers relayed messages across countries in the 1800s. Operators would set the arms at different angles, and the next tower down the line would read the message and pass it along. A message could travel hundreds of miles in just hours, which seemed miraculous before the telegraph.

Railroads use semaphore signals too: mechanical arms that move up or down to tell train engineers whether the track ahead is clear or blocked. Though modern trains mostly use colored lights, you can still spot old semaphore signals standing beside tracks in some places.