Before the days of radio, flags were an important means of communication between ships. If you were not close enough to shout a message to another ship, you could use signal flags to send your message. In 1855, an International Code adopted special flags to help mariners communicate.

Flags are particularly important at sea, where than can mean the difference between life and death, and consequently where the rules and regulations for the flying of flags are strictly enforced. A national flag flown at sea is know as an ensign. a courteous, peaceable merchant ship of yacht customarily flies its ensign (in the usual ensign position) together with the flag of whatever national it is currently visiting at the mast (knows as a courtesy flag). To fly one's ensign alone in foreign waters, a foreign port or in the face of a foreign warship traditionally indicates a willingness to fight, with cannon, for the right to do so. This custom is still taken quite seriously by many naval and port authorities and is readily enforced in many parts of the world by boarding, confiscation, and other civil penalties. In some countries yacht ensigns are different from merchant ensigns in order to signal that the yacht is not carrying cargo that requires a customs declaration. Carrying commercial cargo on a boat with a yacht ensign is deemed to be smuggling in many jurisdictions. There is a system of International maritime signal flags for numerals and letters of the alphabet. Each flag or pennant has a specific meaning when flown individually.

'Dressing Ship' is done with the International Code Signal Flags. Officer's flags, club burgees and national flags are not used. The ship is dressed at 0800 and remains dressed until evening colours (an anchor only, except for a vessel's maiden or final voyage). The Yacht Ensign is hoisted at the stern staff and the Union Jack may be displayed at the bow staff. A rainbow of International Code flags are then arranged from the waterline forward to the waterline aft. Flags and pennants are bent on alternatively. It is good practice to follow a sequence of two flags, one pennant, two flags, one pennant etc. The sequence of flags can be any order but the following is recommended to give a harmonious colour pattern.

Starting forward: AB2 - UJ1 - KE3 - GH6 - IV5 - FL4 - DM7 - PO THIRD REPEATER - RN FIRST REPEATER - ST ZERO

AC - I am abandoning my vessel

BR - I require a helicopter

AN - I need a doctor

CD - I require immediate assistance

DV - I am drifting

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

RU - Keep clear of me; I am maneuvering with difficulty

SO - You should stop your vessel instantly

UM - The Harbour is close 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

EF - SOS / MAYDAY has been cancelled

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

QT - I am going astern

QQ - I require health clearance

QU - Anchoring is prohibited

QX - I request permission to anchor

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