International Morse Code Standard: ITU-R M.1677-1

By the Learn Morsy Content Team · Reviewed by Enes Vural (Morsy founder) · Published July 23, 2026 · Verified against ITU-R M.1677-1 (2009)

Most people who say “Morse code” mean International Morse Code without knowing there is another version. The original American Morse, invented by Samuel Morse and Alfred Vail in 1836, is a different code with different letter patterns. It ran the telegraph systems of the 19th century and then quietly died over the first half of the 20th, replaced everywhere by the International standard that ham radio operators, pilots, and mariners still use in 2026.

Here is what International Morse Code actually is, what the ITU standard specifies, how it differs from the American version most tutorials never mention, and why the distinction matters if you learn Morse for real use.

What International Morse Code is

International Morse Code is the standardized worldwide version of Morse defined by ITU-R Recommendation M.1677-1 (2009). Every letter, digit, punctuation mark, and procedure signal on our Morse code chart comes from that document. When any modern source says “Morse code” without qualification, this is what they mean.

The standard covers three parts:

• Symbol table. 26 Latin letters (A-Z), 10 digits (0-9), 16 punctuation marks (period, comma, question mark, apostrophe, colon, semicolon, and others), plus regional accented characters for European languages.
• Procedure signals. Prosigns like AR (end of message), SK (end of contact), KN (named station only), and the distress signal SOS.
• Timing rules. The 3:1 dit-dah ratio, the 1/3/7 unit gap sequence, and the PARIS reference word for WPM calibration.

Every ham radio contact, every VOR aviation beacon identifier, and every maritime SOS uses these three parts. There is no room for regional variation on core content, which is exactly what makes International Morse work as an international standard.

The path from American Morse to International

American Morse was the first working telegraph code. Samuel Morse and Alfred Vail finished it around 1836, patented it in 1840, and put it into commercial use with the Washington-Baltimore telegraph line in 1844. It ran the entire US telegraph network for over 80 years.

The problem was that American Morse used internal gaps inside some letters. C was dit-dit space dit (two dits, a short pause, then another dit) rather than the modern dah-dit-dah-dit. O was dit space dit (two dits with a gap between them). Over a clean telegraph wire operated by a trained clerk, those internal gaps were reliable. Over a noisy radio channel, they were ambiguous. Was that pause a letter gap, a symbol gap, or static?

Continental European telegraph networks adopted a cleaner variant called Continental Code (later renamed International Morse) in the 1850s. The 1865 International Telegraph Convention in Paris made it the standard for all international traffic. American railroads and the Western Union network held out on the older code until radio pushed them.

Why American Morse died

The 1912 Radio Act, passed after the Titanic disaster, required US maritime and government radio stations to use International Morse for compatibility with the rest of the world. That was the first big push. The American Radio Relay League adopted International for amateur radio operations in 1927. The last commercial landline telegraph offices closed by the late 1960s. The Wikipedia Morse code article has a good timeline of the transition years.

By 1970, American Morse survived only in preservation clubs like the Morse Telegraph Club. Today it is a historical curiosity, not a working code. If you learn Morse in 2026 from any modern source, including our Koch method guide, you are learning International Morse.

What changed between American and International Morse

Twelve letters have different patterns. The rest of the alphabet is identical. Here are the ones worth knowing about if you ever look at old telegraph documentation.

The American O in particular is notorious. Old-timers tapping dit-space-dit and expecting the receiver to hear “O” is one of the classic reasons the code failed over noisy channels. On a clean telegraph wire the gap was audible. On shortwave radio between London and Cape Town the gap disappeared into fading.

What ITU-R M.1677-1 actually specifies

The 2009 revision of the ITU-R M.1677 document covers exactly six sections:

1. Preamble and scope. International Morse is defined for use in radiotelegraphy, terrestrial and space services, alerting, and identification of radio stations.
2. Symbol tables. All 26 letters, 10 digits, and 16 punctuation marks with their dit-dah patterns.
3. Timing rules. Element durations, gap ratios, PARIS reference word.
4. Procedure signals. The complete prosign list operators can use.
5. Regional and accented character extensions. For European languages.
6. Speed and quality guidelines. For amateur, aviation, and maritime use.

The whole document is under 20 pages. Compare it to any modern telecom standard and it is remarkably compact. That is deliberate: the point of International Morse is to be simple enough to learn and consistent enough to send by hand at 5 WPM or by machine at 50 WPM without any change in specification. See our Morse timing rules post for the applied version of the timing section.

The 3:1 timing ratio and the PARIS reference

Every timing rule in International Morse references the dit-length as one time unit. A dah is three units. Gap inside a letter is one unit. Gap between letters is three units. Gap between words is seven units. Nothing else.

The word PARIS is used as the reference for WPM calibration. Sending PARIS at 1 WPM takes exactly 60 seconds under standard timing. This calibration was picked because PARIS contains a balanced mix of dits, dahs, and gaps, which averages out the timing math. Any WPM value on our Morse translator or in any commercial CW radio is calibrated against this word.

Why did the ITU pick PARIS? History and simplicity. A short, universally recognized city name that happens to have the right symbol distribution. Alternatives like CODEX (used in some older documents) do not average out cleanly, so PARIS won by consensus.

Regional and accented character extensions

The standard includes patterns for accented characters used in French, Spanish, German, Swedish, and other European languages. Most operators never need them. They exist for completeness and for languages where the accented character is common enough to matter.

If you plan to operate on the air in a language that uses these characters, learn them. Otherwise you will never encounter them in practice. Ham radio call signs never use accented letters, contest exchanges never use them, and pilots reading VOR IDs never encounter them.

International Morse in aviation

Every VOR (VHF Omnidirectional Range) and NDB (Non-Directional Beacon) in the world transmits its three-letter identifier in International Morse continuously. Pilots tune the receiver to the beacon frequency and listen to verify they are on the correct station.

The identifier is sent at 5-7 WPM, slowly enough that a pilot who knows the alphabet cold can copy it under cockpit workload. The requirement is written into ICAO Annex 10 (radio navigation), which references ITU-R M.1677 for the symbol set. Every instrument-rated pilot has to pass a knowledge test on decoding these Morse IDs.

This is one of the reasons Morse is not going away. Aviation NAV infrastructure worldwide depends on it, replacement systems cost billions, and the existing hardware works fine.

International Morse in maritime

The Global Maritime Distress and Safety System (GMDSS) replaced Morse-based distress communication in 1999. Commercial ships no longer need a Morse-trained radio operator on board. But the SOS pattern (three shorts, three longs, three shorts) is still recognized under international maritime regulations as a valid distress signal on any medium, including radio, light, sound, and flags.

Our full history of SOS is in our SOS post. The short version: SOS predates GMDSS by 90 years and survived because it works on any channel with an on-off switch and no equipment beyond the switch itself. GMDSS covers the routine case. SOS covers the disaster where nothing else works.

International Morse in amateur radio

All amateur radio CW (continuous wave) operation worldwide uses International Morse. Every ITU region uses the identical standard, so a US ham working DX (long-distance contact) with a Japanese operator uses the exact same 26 letters and 10 digits at both ends. That is the whole point of the international standard.

The ARRL learning-Morse-code resources walk through the beginner-to-fluency path for ham CW. Our complete Morse learning guide covers the same territory with a Koch/Farnsworth focus and a 3-4 week roadmap.

Ham CW remains active in 2026 because it is the most efficient mode for weak-signal work. A 5-watt CW transmitter can span oceans on a good day. Voice modes need 100+ watts for the same reach. This physical advantage is what keeps CW alive on the ham bands decades after every other original radio mode migrated to digital.

Common misconceptions about International Morse

“Morse code is just dots and dashes.” True, but the specification is much stricter than that. Two systems using dots and dashes are not automatically compatible. American Morse and International Morse both use dots and dashes and produce different letters. The standard matters.

“There is only one Morse code.” False. There is one dominant standard (International, ITU-R M.1677-1) and one historical predecessor (American, extinct in practical use). Some sources mention wabun (Japanese kana Morse), which is a separate code layered on the same dit-dah system for Japanese katakana characters.

“The international standard has not changed in a century.” False. The core letter patterns are unchanged since the 1938 revision, but the ITU has revised the surrounding document multiple times: 1938, 1949, 2004 (as M.1677), and 2009 (M.1677-1). The revisions cover accented characters, procedure signals, and timing precision.

“American Morse is a variant of International.” No, American came first. International replaced it. The variants list runs American to International, not the other way around.

What learners should take away

If you learn Morse in 2026, you are learning International Morse. Every app, every chart, every online translator including ours uses ITU-R M.1677-1. Historical curiosity aside, American Morse is not a working code and not worth learning unless you specifically want to preserve landline telegraph tradition.

The letters and digits are what our Morse code numbers post covers in detail. The prosigns are on the chart with usage notes. The timing rules are in our how-Morse-works post. Everything you touch as a modern Morse learner traces back to the International standard.

FAQ

What is International Morse Code?

International Morse Code is the modern standardized version of Morse, defined by ITU-R Recommendation M.1677-1 (2009). It specifies 26 letters, 10 digits, 16 punctuation marks, procedure signals, and the exact timing ratios between dits, dahs, and gaps. Every ham radio contact, aviation NAV beacon, and maritime distress signal uses this standard worldwide.

What is the difference between American Morse and International Morse?

American Morse (also called Railroad Morse) was the original code developed by Samuel Morse and Alfred Vail in 1836. It uses variable letter lengths and internal gaps within letters like C, O, R, Y, and Z. International Morse, adopted at the 1865 Paris ITU convention, replaced those with cleaner patterns designed for radio. American Morse survived only in preservation clubs after the 1970s.

When was International Morse Code standardized?

The core specification was adopted at the 1865 International Telegraph Convention in Paris. It was revised in 1938, again in 1949, then formalized as ITU-R Recommendation M.1677 in 2004 and revised as M.1677-1 in 2009. The 2009 revision is the current standard used worldwide.

Does International Morse Code include letters with accents?

Yes. ITU-R M.1677-1 includes regional extensions for accented characters used in French, Spanish, German, and other languages. Examples include Ä (dit-dah-dit-dah), Ö (dah-dah-dah-dit), Ü (dit-dit-dah-dah), and É (dit-dit-dah-dit-dit). These are documented in the standard even though most operators rarely send them.

Is International Morse Code still used in 2026?

Yes, in three active domains. Amateur radio operators use it for CW (continuous wave) contacts worldwide. Aviation VOR and NDB navigation beacons transmit their identifier in International Morse continuously. Maritime SOS is still recognized under international regulations even though the GMDSS replaced Morse-based maritime distress in 1999.

What does ITU-R M.1677-1 actually specify?

The document specifies the dit-dah patterns for 26 Latin letters, digits 0-9, 16 punctuation marks, and a set of procedure signals (prosigns). It also defines timing rules: a dah is three time units, a dit is one, letter gaps are three units, word gaps are seven. All ratios reference the dit as the base unit.

Why did American Morse die out?

American Morse required internal gaps within letters, which worked over telegraph wire but caused ambiguity over noisy radio channels. The 1912 Radio Act pushed maritime and government stations to International Morse. The ARRL followed for amateur radio in 1927. By the 1970s, American Morse survived only in landline telegraph preservation clubs.