How did a Roman camp tell friend from foe at night?

A Roman watchword let sentries tell friend from foe in the dark, and a new one was given out every day so a spy couldn’t learn it and slip in. That meant every unit in the camp had to get the new word before dark. A single unit without it could leave a gap in the camp’s guard. The historian Polybius described how a camp managed it. At sunset a soldier from the unit camped farthest away walked to the tent of the tribune, the officer in charge of the camp, and carried the word back, written on a wooden tablet. Before witnesses he passed it to the next unit, which passed it on in turn, back up the line, until the last brought the tablet to the tribune before dark.

The returning tablets were receipts. Each was marked for the line of units it traveled, so if every one came back, the tribune knew the word had reached every unit. If one was missing, he knew which line to question, and whoever had held it up was punished. It is the same problem any office faces when a shared password changes: telling everyone, and knowing that everyone was told.

The English words come from these ideas. A “watchword” was the word for the watch, the guard on duty; a “password,” first recorded in 1798, was the word that let you pass.