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How to Read and Write Flag Semaphore
Heshan Fernando
Co-founder & COO
Flag semaphore is a system for sending letters using two flags held at different angles. It’s largely obsolete, it’s still taught in navies and scouting, and it comes up in puzzles, escape rooms and the occasional maritime museum.
It’s also a genuinely elegant design, and understanding why it works the way it does makes it much easier to learn.
How the system encodes letters
Each arm can be held at one of eight positions, spaced 45 degrees apart around a clock face — think of them as low, low-diagonal, horizontal, high-diagonal and vertical, on each side.
Two arms, eight positions each, gives 64 combinations. Remove the ones where both arms occupy the same position and the ones that are physically awkward, and you’re left with enough distinct signals for 26 letters plus a set of control signals.
The letters are assigned in a systematic order rather than randomly: A through G use one arm in the lowest position while the other moves round, H onward advances the first arm, and so on. Learning that structure is far easier than memorising 26 unrelated shapes.
Positions are described from the signaller’s point of view. An observer facing the signaller sees them mirrored, which is the single most reliable source of confusion when learning. If a diagram says “low left”, that’s the signaller’s left.
Numbers reuse letters
There aren’t separate signals for digits. Instead, the letters A through K double as the numerals 1 through 0, and a numeric sign switches the receiver into number mode. A letters sign switches back.
This is efficient and fragile. Miss the numeric sign and a receiver reads a string of letters where a number was intended — and since both are valid, nothing looks wrong until the message makes no sense.
That’s why control signals matter as much as the characters, and why any transcription of a semaphore message needs to show where the mode switches happened.
| Signal | Purpose |
|---|---|
| Numeric sign | Following characters are digits |
| Letters sign | Return to letters |
| Attention | Ready to send |
| Error | Cancel the previous group |
Why it survived as long as it did
Semaphore needs no equipment beyond two flags and no power. It works when radio is unavailable, when radio silence is required, or when equipment has failed — which is exactly why navies kept teaching it long after radio was universal.
Its range is line of sight, its speed is roughly a few words a minute in skilled hands, and it degrades gracefully: a partially misread message is usually recognisable as garbled rather than silently wrong.
Common mistakes to avoid
- Reading positions from the observer’s perspective, which mirrors every letter.
- Forgetting the numeric sign, which turns a number into a nonsense word.
- Confusing the diagonal positions, which are the ones most easily misread at distance.
- Assuming semaphore and Morse are interchangeable — they encode differently and semaphore has no timing element.
- Learning the letters as 26 separate shapes rather than as a systematic progression.
How to do it with Semaphore Flag Translator
The Semaphore Flag Translator converts in both directions with the control signals shown.
- Enter the text; semaphore covers letters and numerals, not punctuation.
- Read positions as clock angles from the signaller’s own perspective.
- Note where the numeric sign appears — that’s where a message gets garbled.
- Translate back from positions to check your reading.
The International Code of Signals covers the wider maritime signalling system semaphore sits within. Other education tools are in the tools directory.
Frequently asked questions
Is semaphore still used?
Rarely, and mostly as a backup where radio silence matters or equipment has failed. Navies still teach it, and it appears in scouting and maritime training.
Why do numbers reuse letter positions?
Because there are only so many distinguishable arm positions. A numeric sign switches the receiver into number mode and a letters sign switches back.
Whose left is left?
The signaller’s. Positions are described from the sender’s point of view, so an observer sees them mirrored — a reliable source of confusion when learning.
Final thought
Learn the systematic order rather than 26 shapes, and always note whose left you’re reading. Both halves of that sentence account for most beginner errors.