The international signal flags

Ships use a set of colored flags to spell out messages, one flag for each letter. Each flag also has its own meaning when flown alone, like Alfa for "diver down." The designs use red, blue, yellow, black and white in stripes, crosses, squares and checks so they can be told apart at a distance.

Puzzle caches use them as a picture cipher, sometimes drawn, sometimes as colored squares or real flags on a pole. Tap each flag in order to spell the message.