Maritime Signal Flags.
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.
It is a component of the International Code of Signals (ICS).
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 and (more importantly) to warn other vessels to keep clear to avoid endangering the diver(s) with their propellers.
- 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” (for more details see Race Signals).