Maritime Flags

Maritime signal flags, not to be confused with Flag Semaphore, are a method of communicating between two ships via a set of flags with standardized meanings. The most common of these sets of flags is the International Code of Signals (ICS), which is used by most navy vessels as well as many pleasure and commercial vessels.

In puzzles, the ICS is also the most commonly-used set of signal flags, as it contains all letters in the Latin alphabet and every digit from zero to nine. This allows for full use as a substitution cipher, in addition to their use in expressing specific nautical messages. The use of only 5 colours also allows them to be depicted easily in multiple mediums, and to be described easily using plain English or heraldic terminology, making the ICS in particular a very versatile puzzle element.

Background
Flags have been used for communication at sea since at least the 16th century. However, the complexity and nuance of this communication has changed drastically since the golden age of piracy.

16th to 18th Century Use
In the early days of inter-ship communication, signalling was primarily done through a process of placing flags in particular positions on the ship. The goal of doing was usually to express the state of the ship, especially in terms of willingness to interact with other vessels. For organized fleets, the flags used were usually flags unique to the kingdom of origin or the fleet itself. This process was also adopted by pirates during the golden age of piracy (1650s to 1730s), where a colorless or borrowed fleet flag would be raised to fool other ships into thinking the pirate ship was safe or friendly until it was too late (and a jolly roger or other identifying flag was raised).

In the 17th and 18th centuries, multiple more sophisticated signal systems were developed, with mixed results. The British Royal Navy tried out a different position-based system in the early-to-mid-17th century, but switched to something resembling modern flag signalling soon after. In 1790, they swapped once again to a numerical system and a designated code book containing many possible phrases. The French navy almost adopted a similar system using ten solid colored flags and a massive code book. It was not officially adopted, but was adapted by Ignace Chappe into what would become the optical telegraph. A second French system was published in the mid-18th century, but was similarly not widely adopted.

The most successful innovation from this period was by Home Popham of the British Royal Navy, who established a system of 40 flags covering the alphabet (minus J), the 10 digits, and five operation flags (substitution, numbers following, message start, message end, and 'yes'). These flags could be combined to form individual words via a codebook, or to spell words using the alphabetical flags. This code is one of the systems that is most similar to modern-day maritime signalling. It has one major drawback, particularly when it came to conveying shorter words that were not in the code book, as each individual letter would require one or two flags, followed by an 'end of message' flag, meaning that longer messages had to be done via multiple rounds of hoisting and lowering the sets of flags.

Puzzle Application
While pre-ICS maritime signalling systems have the potential for use in puzzles, they have yet to be used in any major puzzle hunts. The ICS, however, has been used many times, in multiple different ways.

Use as an Alphabet
While not necessarily the most space- or time-effective use of the flags at seas, having each flag represent a letter in a message is a common sight in puzzles that use them at all. The ICS flags in particular each have a letter from A-Z associated with them, along with the appropriate NATO Phonetic Alphabet letter (or the allied military phonetic alphabet prior to 1956), meaning that they can represent any string of letters a writer could want.

Use as a Language
Less commonly used in puzzles are the meanings of flags when presented individually. Since each conveys a particular nautically-helpful message, writers could theoretically use them to represent those messages, and extract from them. However, it's more common to do this in the opposite direction (i.e. present the messages to clue the flags and then extract via the alphabet mapping), as there is no standard for spelling or wording of the messages, only the overall meaning.

Beyond those single-flag messages, the ICS flag system can also be used to express other things when paired with numbers or other letter flags. These pairing are well-defined and updated to 21st century standards as seen in the International Code of Signals guidebook. These signals include medical signals, location/time/speed signals, and distress signals, among other, more mundane messages.

Identification
Since many puzzles will use maritime signal flags outright, simply being able to recognize that an individual flag is one of the ICS's flags will be enough to complete the identification step. A solver doesn't even necessarily need to memorize the patterns to be sufficiently familiar with them to perform a quick ID. Knowing that all ICS alphabet flags are usually presented as squares (or a similarly-sized, swallowtail shape for A and B), and all of them pull from the same set of five colors (red, blue, yellow, white, black) should be enough.

When a puzzle chooses not to use the flags directly, there are other things to be on the lookout for. If a puzzle is mostly text-based, watch for phrases that sound like short messages, as the puzzle may be representing flags by their non-letter meanings. If a puzzle is more image- or other visual media-based, watch for the aforementioned five colors, as they may be used to represent the flags through various combinations. Regardless of the basis of the puzzle, certain words in the flavortext or title can also allude to their use. These include nautical terminology (starboard, port, etc.), synonyms for ship, and the words 'signal' and 'flag'.

Solving
When puzzles use maritime signal flags as just an alphabet replacement, 'solving' that part of the puzzle is extremely straightforward, as it only requires translating the flags back into the letter counterparts. This is true regardless of the way that the flags are presented (as text descriptions, as images, as abstract concepts, etc.), since most puzzles will have the more difficult part be something outside of the direct translation.

The one possible known exception to this is if the flags are used as an alphabet substitution, but ignoring the proper mapping of flag to letter. In this case, refer to the strategies for cryptograms.

Played Straight

 * Cambridge Ante (MITMH 2006) - Simply requires translating flags to letters (and some other steps but the use of flags is fairly straightforward)

Notable Twists

 * English Expectations (MITMH 2011) -
 * Vertexillonomy (MITMH 2013) - Rather than showing the flags directly, this puzzle used its core concept to represent flags in vertex graph form.