Flag Semaphore

Flag Semaphore (often referred to as simply Semaphore) is a communications-based Decryption element, used for maritime communications in the 19th century. Semaphore is one of the most commonly-used ciphers in puzzlehunting, along with Morse Code, Braille, and five-bit Binary.

Background
See also: Flag semaphore

Puzzle Applications
Semaphore's main draw in puzzling comes in its role as an extraction element. Semaphore extractions need two directions (and potentially an anchor point defining "up", which is normally the upward direction on a computer monitor), which can be defined any number of ways: lines between two endpoints and an anchor for the center, "pointers", compass directions, actual movement, etc.

This form of extraction can come clued or unclued. The go-to example of semaphore being used as an extraction is what has become known as clock semaphore, where setting a clock to a specific time makes the hands form a semaphore letter.

Spotting the usage of Semaphore
The usage of Semaphore is sometimes clued using the title, flavortext, or other parts of the puzzle—especially common is the word "flag", though other flag-related words could show up as well.

Regardless of whether the flavortext hints at semaphore or not, one should be suspicious of anything that could be construed as a pair of cardinal/intercardinal directions. A pair of points with a known anchor point is easy to convert to semaphore, as are a pair of line segments sharing one endpoint. Even the presence of something that divides the space around a point into eight directions (such as a square grid) may be a tell for semaphore, though in these cases it's usually easier to tell via the flavortext.

Translating Semaphore
Many semaphore charts and interpreters can be found online. To save time on transcription, however, one may wish to memorize semaphore. For the sake of explanation, this section labels each direction with a number, starting with 1 for down and proceeding clockwise (so that left is 3 and right is 7).

Semaphore operates on a "rotary dial" system: starting from 1-2 (A), move the "second" (further clockwise from down) flag by one space to obtain the code for the next letter. After the "second" flag reaches 8, move the "first" flag by one space and the "second" flag to the space directly clockwise from the first flag (so after 1-8 (G) would be 2-3 (H)). Continue in this manner for the rest of the alphabet. The letters JVWXZ, however, will be skipped; they get the codes near the end (starting from 5-7 (J) and continuing in order). These account for 26 of the 28 possible configurations of two distinct directions; of the remaining two, 4-8 stands for "Cancel" and 5-6 stands for "Number" (which converts A-I to digits 1-9 and K to 0 until receiving a "J" signal).

Of course, rederiving semaphore in this way is rather tedious; as such, mnemonics may help with accessing the right part of the alphabet. In particular, N (2-8) and U (4-6) have flag positions reminiscent of their lowercase forms, while D (1-5) and R (3-7) are both straight lines and stand for directions indicated by one of their flags.