International Morse code uses only two signal lengths — a dot and a dash — and is the only Morse standard recognized today by the ITU. American Morse code, the original system developed by Samuel Morse and Alfred Vail in the 1830s and 1840s, used three mark lengths and different character encodings. International Morse replaced American Morse as the global standard during the early 20th century, driven largely by the needs of wireless radio.
Why Two Different Codes Existed
Samuel Morse and Alfred Vail designed their code for the US landline telegraph. Many characters in their system contained internal pauses — a short silence within the character itself — in addition to the dots and dashes. This worked on a stable wire circuit where an operator could hold the key open for a longer mark without ambiguity.
When wireless (radio) telegraphy emerged in the late 1890s and early 1900s, this design created problems. Radio signals fade, skip, and suffer interference. An internal pause within a character could be misread as the gap between two letters, corrupting the message.
Meanwhile, European telegraph operators had developed Continental Morse (the precursor to International Morse), which used only two mark lengths and no internal pauses. It was simpler and more robust over imperfect circuits.
Key Structural Differences
| Feature | American (Landline) Morse | International Morse |
|---|---|---|
| Number of mark lengths | 3 (dot, dash, long dash) | 2 (dot, dash) |
| Internal pauses within characters | Yes (some characters) | No |
| Gap ratios standardized | No ITU standard | Defined in ITU-R M.1677-1 |
| Current official status | Obsolete | In force (ITU) |
| Primary use era | 19th–early 20th century | 20th century–present |
How Character Encodings Differ
American Morse encoded several letters differently from International Morse. The letter O in American Morse was two dots separated by a longer internal space (. .), which an International-trained ear hears as “EE”; International Morse encodes O as three dashes (—). This means text transmitted in one code is not intelligible to an operator trained in the other.
The letter C in American Morse was two dots, a pause, then one more dot (.. .) — an internal pause that has no equivalent in International Morse, where C is -.-.
Some numbers also differ. American Morse numbers used shorter sequences, while International Morse numbers each use exactly five elements (all dots for 5, all dashes for 0, and mixtures for 1–4 and 6–9).
International Morse: The Standard Today
International Morse code is defined in ITU-R Recommendation M.1677-1, approved in its current form in 2009. Its timing ratios are fixed: a dash equals three dot-lengths, the gap between elements in one letter equals one dot-length, the gap between letters equals three dot-lengths, and the gap between words equals seven dot-lengths.
The ITU notes in M.1677-1 that versions of the code have been in use since 1844 and that it continues in amateur and amateur-satellite services.
The American Morse code alphabet (partial comparison)
| Letter | American Morse | International Morse |
|---|---|---|
| A | .- | .- (same) |
| C | .. . (dot dot, space, dot) | -.-. |
| F | .-. | ..-. |
| J | -.-. | .— |
| L | one long dash, about twice a normal dash | .-.. |
| O | . . (dot, space, dot) | — |
| P | ….. | .–. |
| R | . .. (dot, space, dot dot) | .-. |
| Y | .. .. (dot dot, space, dot dot) | -.– |
| Z | … . (dot dot dot, space, dot) | –.. |
| S, T, E, H, I | same as International | same |
The “space” inside C, O, R, Y and Z is a longer internal gap, which is why a careless operator’s R could be read as “EI”. According to the W5SF guide to American Morse, 15 letters are identical in both codes: A, B, D, E, G, H, I, K, M, N, S, T, U, V and W. American Morse never had an ITU-style timing standard, so the length of the long dash and the internal gaps varied between operators.
Why It Matters Today
If you are learning Morse code for amateur radio, emergency preparedness, or recreation, learn International Morse. That is what ITU-R M.1677-1 defines, what the ARRL teaches, and what the Morse code translator on this site uses.
American Morse survives in railroad museum demonstrations and historical telegraphy groups, but it is not used in any current licensed radio service.
For more background, see history of Morse code and does anyone still use Morse code.