International maritime signal flags are flags that are used to communicate with ships.

The International Code of Signals is the principal system of flags and codes.

Flags can be used to spell out a message.  This can be done because each flag represents a letter of the alphabet.

Various individual flags have a standard meaning attached to them – for example, when the “A” flag is raised it indicates the boats inability to move from their present position because there is a diver underwater.

When more than one flag is displayed, they for a code word.  This word can be looked up in a code book.

AC – I am abandoning my vessel.
AN – I need a doctor.
BR – I require a helicopter.
CD – I require immediate assistance.
DV – I am drifting.
EF – SOS/MAYDAY has been canceled.
FA – Will you give me my position?
GW – Man overboard. Please take action to pick him up.
JL – You are running the risk of going aground.
LO – I am not in my correct position: used by a light vessel.
NC – I am in distress and require immediate assistance.
PD – Your navigation lights are not visible.
PP – Keep well clear of me.
QD – I am going ahead.
QQ – I require health clearance.
QT – I am going astern.
QU – Anchoring is prohibited.
QX – I request permission to anchor.
RU – Keep clear of me; I am maneuvering with difficulty.
SO – You should stop your vessel instantly.
UM – the Harbour is closed to traffic.
UP – Permission to enter Harbour is urgently requested. I have an emergency.
YU – I am going to communicate with your station by means of the International code of signals.
ZD1 – Please report me to the Coast Guard, New York
ZD2 – Please report me to Lloyds, London.
ZL – Your signal has been received but not understood.