Countersign

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Countersign is a military term for a sign, word, or any other signal previously agreed upon and required to be exchanged between a sentry or guard and anybody approaching his or her post. The term usually encompasses both the sign given by the approaching party as well as the sentry's reply. However, in some armies, the countersign is strictly the reply of the sentry to the password given by the person approaching. A well-known sign/countersign is that used by the Allied forces on D-Day during World War II: the challenge/sign was "flash" and the answer/countersign "thunder".

The signs are often worked into a sentence in some way, nominally to obfuscate which words are key in case there are spies nearby. An example would be the sign "blue" and the countersign "moon". The approacher might say, "I'm feeling blue today... got any ideas of how I could cheer up?" The sentry could reply with, "Well, you could always moon somebody." In actuality it would be easy to figure out what the words are, although this method is better than shouting the words plainly. Presumably this effect is greater multiplied when the listener doesn't speak the language.

Countersigning also means writing a second signature onto a document. For example, a contract or other official document signed by the representative of a company may be countersigned by his supervisor to verify the authority of the representative. Also, a money order or other financial instrument may be signed once upon receipt, then signed again by the same person when presented for payment, as an indication that the bearer is the same person who originally received the item, and not a thief who has stolen the item before it could be carried to the place where it was to be presented.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.