"Countersign (military)"@en . "In military terminology, a countersign is 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\", the password \"thunder\", and the countersign (to challenge the person giving the first code word) \"Welcome\"."@en . . . "In military terminology, a countersign is 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\", the password \"thunder\", and the countersign (to challenge the person giving the first code word) \"Welcome\"."@en . . . .