Morse code

What Morse code is

In Morse, each letter and digit is a sequence of one to five signals of two lengths, the dot and the dash, with a dash conventionally three times as long as a dot. Written down, a dot is a full stop and a dash a hyphen. The letters are separated by a space and the words by a longer gap, usually printed as a slash. The common letters have the shortest patterns, E and T with one signal each, I, A, N and M with two, which was a deliberate choice to make English messages short to send.

The code is named for Samuel Morse, whose electric telegraph of the nineteenth century needed a way to send letters as pulses down a wire; the version in use today is the international form, standardised later, which differs from Morse’s original in several letters. For a century it carried telegrams, ship-to-shore traffic and distress calls, the familiar SOS being three dots, three dashes, three dots, and it is still learned by amateur radio operators and still appears in puzzle books and, in its fractionated form, on Codebusters papers.

Decoding is lookup. Split the message at the slashes to find the words, then read each word one group at a time, writing the letter under its group. Most students know a handful of patterns cold, E, T, S, O, and a message with those filled in often reads before the rest is looked up. A table with the letters in alphabetical order is slow to search by pattern; a table arranged by pattern length, or by first signal, finds a group much faster, and drawing one of those is worth the two minutes.

The mistakes are mostly in the grouping. A dot read as a dash, or the reverse, changes the letter outright, and a space missed between two groups fuses two letters into a pattern that is often a real letter too, so the error is not obvious until the word fails to read. Counting the groups in a word before decoding any of them, and checking the count against the decoded word’s length, catches most of these. Writing the letters directly under their groups keeps the alignment honest.

How to solve Morse code

Each letter is a short group of dots and dashes from a fixed table — E is a single dot, T a single dash, S three dots, O three dashes. A space separates one letter’s group from the next, and a slash marks the end of a word. There is no key to find; the work is reading the groups off the table without losing your place.

1. Split the message at the slashes first. Each piece between slashes is one word.
2. Inside a word, each group of dots and dashes with a space either side is one letter. Count the groups: that is the word’s length.
3. Look up each group on the Morse table and write its letter underneath. Start with the one-signal letters, E (.) and T (-), and the three-of-a-kind, S (...) and O (---).
4. Read the word back. If it is not English, the usual slip is a dot read as a dash, or a space missed so that two letters ran together.
5. With no table to hand, write out the letters you know cold first (E, T, A, I, N, S, O) and work the rest out from the groups that repeat.

A worked example

Decode ... --- ..- -. -.. / .. ... / .- / .-- .- ...- .

Three slashes, so four words. Then one group at a time:

... → S   --- → O   ..- → U   -. → N   -.. → D
.. → I   ... → S
.- → A
.-- → W   .- → A   ...- → V   . → E

Plaintext: SOUND IS A WAVE

Morse in Codebusters

Among the Science Olympiad rule sets OlyPrep has read, plain Morse is not itself a Codebusters question at the elementary or Division B level. Division B names the Fractionated Morse cipher, in which the message is first written in Morse with separators, then cut into groups of three signals, and each group replaced by a letter from a keyed alphabet; the Division B reference sheet OlyPrep has read carries a Morse table for that purpose. The Milpitas elementary list does not include Morse.

That makes Morse a foundation rather than a question. A student who can read Morse groups fluently has done the easy half of Fractionated Morse before the hard half begins, and in that cipher the usual scoring applies: by the letters of the decoded message, with a decoded quotation of up to two wrong letters commonly counted as solved. Morse also turns up in puzzle hunts and escape rooms, where the table is rarely provided and the common letters have to be known.

The lab below encodes a science sentence in standard Morse, with the letters separated by spaces and the words by slashes, and the key line says so. The easy setting uses a short sentence and the hard setting a long one, so the practice scales from a few groups to several dozen. There is no table in the lab; the card above has the method and a worked example, and a student who wants the full table draws it once and keeps it beside the screen.

Practice Morse code

The lab below is set to Morse code alone. Pick a difficulty, generate a challenge, decode it, then check your answer or reveal the solution. It runs in your browser, needs no account, and is unlimited on every OlyPrep plan.

Where Morse code is tested

None of the Science Olympiad rule sets OlyPrep has read names plain Morse on its elementary or Division B Codebusters list; Division B names the Fractionated Morse cipher, which is built on it. It is in the open lab as the step before that cipher.