Scytale

The sender wound a strip spirally around a staff and wrote the message lengthwise. Unwound, the strip showed a jumble of letters; only around a staff of the same thickness did the text line up again. No letter is replaced, only moved, which is what makes the scytale a transposition cipher rather than a substitution cipher.

Plutarch describes it in his life of Lysander, though several historians doubt it was used for secrecy at all; it may just as well have been a way of authenticating dispatches. As a cipher it is weak: try staffs of different girth, or simply count columns, until words appear.