Before the invention of the radio, the only way mariners could pass messages from one ship to another was by means of visual signals. Strips of coloured cloth would be hoisted up the mast to send a predetermined signal which saved considerable time while afloat. These pieces of cloth have evolved into the distinctive shapes and patterns that make up the flags and pennants used today.

In the present day, most communication between ships is accomplished electronically but flags and pennants are still used to communicate intentions, movements and general information to ships within visual range.