When you've been out on deck, you've probably seen them flapping in the ocean breeze, but how many of you have the faintest notion of what a ship's flags signify? Keep reading, and you can impress your shipmates next time with your knowledge of their arcane symbolism.

Assorted maritime flags that flutter on every ship testify to centuries of seafaring heritage and tradition. According to Captain Tom Thomason, Carnival's director of corporate shipbuilding, a ship's flags are "almost ritual and prescribed." Even today--when sophisticated technologies have supplanted flags as primary communications tools--ships "still follow correct methods, manners and mores concerning them," he said.

We're not only talking about so-called national "flags of convenience"--i.e., by registering ships in countries like Liberia, the Bahamas and Panama, cruise lines save a bundle in corporate taxes, and sometimes are even immunized from regulatory oversight. That's another story.

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We're talking about an array of colorful flags and pennants that fly forward, aft or topside on a ship. These actually speak an ancient and obscure tongue, but a curious or observant passenger can learn a lot by interpreting their iconic language.

Thomason succinctly sums them up this way, from the ship's point of view: "A flag says who I am, what I am and what I'm doing."

From antiquity, when nothing more than a pair of eyes or a set of spyglasses linked ships at sea, sailors used flags to communicate. Flags function the same way today, Thomason says: "Albeit often used differently than they originally were on medieval ships and warships, they serve the same purpose."

The earliest flags were fairly basic: A ship flew its nation's flag to convey "I'm an English, American or Spanish ship," Thomason says. Eventually, specific flags were added for "courtesy," as when a ship is in foreign waters. A Spanish warship sailing into another country's harbor with peaceful intentions also flew that nation's flag "to send a clear sign its intent was innocent." This is not to say that pirates wouldn't occasionally fly a misleading flag as a tactical deception. One can easily imagine a ship flying one flag on approach, then suddenly switching to the Jolly Roger in a surprise attack.

But even though they no longer constitute a ship's first line of communication, flags are not flown arbitrarily. A definite protocol prevails. The logic to flags and where on a ship they fly is almost defined by the wind, Thomason says. Years ago, a ship would fly its national registry flag aft and high, so it would be perfectly visible. Over the years, as the design of new ships moved the main masts forward, so goes that flag--though when a ship is at anchor or in port, it still flies at the stern.

Flags of national registry are not same as flags of owners. The "house" or company flag usually flies on a short foremast at the ship's bow. This flag falls into Thomason's "who I am" category. Princess Cruises' house flag, for instance, depicts four colored triangles--blue and white from the Portuguese national flag and red and yellow from Spain's, colors that reflect the origins of Princess' British parent company, P&O, back in the days when it traded with Spain and Portugal.

Other flags constitute the equivalent of a typesetter's arsenal. Forty colorful and geometric flags comprise a complete communications system: 26 flags denoting each letter of the alphabet, 10 denoting the numerals from 0 through 9, plus an "answering" pennant and three substitutes. The specific meanings of all these flags were defined by international agreement in 1902.

In addition to representing a single letter of the alphabet, a given flag also stands for a short phrase, a shorthand code that describes something, Thomason says. For example, the square red flag--"B" or phonetic "bravo"--communicates caution. "It says 'I'm doing something a bit hazardous'," Thomason explains. This flag flies on the foremast during refueling. Navy ships hoist it when loading munitions.

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Within 20 minutes of entering and leaving port, look for a half-red, half-white flag--"H" or phonetic "hotel"--that pops up on the foremast. It indicates that a harbor pilot is on board. When the pilot departs, the flag comes down.

Likewise, when your ship approaches a foreign port, a square yellow flag on the foremast "sends a signal to harbor authorities... to get ready to come aboard and do their immigration thing," Thomason says.

A flag you may want to particularly notice is the "blue P"--phonetic "pappa," also commonly called the blue peter. Fluttering from a starboard yard, this flag's white square against a dark-blue field says the ship will be pushing off within 24 hours.

Some flags you should hope you never see. The descending blue, white and red squares of the "W"--or phonetic "whiskey"--indicate a medical emergency on board. But should you spot the diagonally divided red and yellow "O" flag--phonetic "Oscar"--look over the rail, Thomason says: It means "man overboard."

And what about those flags often rigged like laundry from stem to stern? It's called "dressing the ship." While the array of alphanumeric flags isn't exactly a pointless potpourri of pennants, the arrangement is customarily strung only for pleasant appearance and shape. Mischievous crew members, though, have been known to engineer some unwanted phrase making here, Thomason admits: "I've had a couple of sailors put up a few with, shall we say, sailor-like language."

Indeed, flags in combination can convey infinite messages. At the simplest level, as when several fly from top to bottom on a mast, they indicate a ship's call letters.

But should you doubt how much of import they impart, Thomason tells this classic story. On the morning of October 21, 1805, as the British fleet sailed into action against the combined fleets of France and Spain, the flagship Victory hoisted the most famous flag signal in naval history. The nine-word message--intended to encourage Nelson's men at Trafalgar--spelled out "England expects that every man will do his duty."

Duly bolstered, they did, and Britain won the battle that changed the face of Europe. Concludes Thomason: "That's why that statue of Nelson stands in Trafalgar Square today, and not the Spanish king."