Flag semaphore is a visual telegraphy system in which a signaller holds a coloured flag in each hand and places the two arms at fixed angles to spell out a message. It was the standard ship-to-ship method before radio and is still part of maritime and scouting training. This encoder converts text into the standard semaphore alphabet, describing the angle of each arm for every letter.
How it works
Imagine the signaller’s body as a clock centre. Each arm can point in one of eight directions: straight down, low (down-diagonal), out (horizontal), high (up-diagonal), and up. The semaphore alphabet is built systematically: A is left-arm low-left / right-arm down, and the pattern rotates one arm at a time through the circle.
This tool stores the canonical left/right angle pair for each of the 26 letters and prints them as readable descriptions such as Left: out-left, Right: down. Words are separated by a blank line so message structure is clear.
The eight arm positions
A semaphore signaller can place each arm in eight clock-like directions, commonly named:
| Position | Clock direction |
|---|---|
| Down | 6 o’clock |
| Low-left / Low-right | 7 or 5 o’clock |
| Out-left / Out-right | 9 or 3 o’clock (horizontal) |
| High-left / High-right | 10 or 2 o’clock |
| Up | 12 o’clock |
With two independent arms and eight positions each, the system has up to 64 possible combinations, enough to encode the 26 letters, a handful of special procedural signals (Ready, Numerals, Annul, Error), plus digits expressed through the Numerals flag.
Sending numbers
Semaphore does not have separate digit symbols. To send a number, the sender first shows the Numerals sign, which indicates that the next signals should be read as numbers. From that point, the letters A through J stand for 1 through 9 and 0 respectively. The receiver watches for the Numerals sign to switch decoding mode. This tool marks digit characters with a note and leaves them in place so you can add the Numerals procedure manually for a complete transmission.
Practical uses today
Flag semaphore is rarely used for serious communication now, but it persists in:
- Scouting and youth groups — many organisations include semaphore as a badge activity and communication exercise.
- Naval traditions — some navies retain ceremonial semaphore competency and use it as a backup when radio silence is required.
- Escape rooms and puzzle design — the visual nature and structured alphabet make semaphore a favourite cipher for puzzle builders.
- Film and theatre — period productions depicting pre-radio naval scenes sometimes feature semaphore for authenticity.
Example
The letter A is Left: low-left, Right: down, and R is Left: out-left, Right: high-right. Spelling a short word shows one labelled line per letter, which a learner can use to practise the arm movements in front of a mirror. Each output line gives both arms, so a complete practice session is possible directly from the encoder output without needing a separate lookup table.