The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships. 

There are various methods by which the flags can be used as signals:

• each flag spells an alphabetic message, letter by letter.
• individual flags have specific and standard meanings; for example, diving support vessels raise the "A flag" indicating their inability to move from their current location because they have a diver underwater.
• one or more flags form a code word whose meaning can be looked up in a code book held by both parties. An example is the Popham numeric code used at the Battle of Trafalgar.
• in yacht racing and dinghy racing, flags have other meanings; for example, the P flag is used as the "preparatory" flag to indicate an imminent start, and the S flag means "shortened course"