Why E is one dit: how Morse code follows letter frequency
E is a single dit because E is the most common letter in English. Morse code gives the shortest codes to the letters used most, so ordinary text takes less time to send: E is one dit, T is one dah, and rare letters such as Q and J get four elements each. The fit is close at the top of the frequency list and loose further down, and both halves of that story are worth knowing if you are learning the code.
Short codes for common letters
With only two kinds of signal, dits and dahs, there are very few short codes to go round. There are 2 codes of one element, 4 of two elements, 8 of three and 16 of four. That makes only 14 codes of three elements or fewer, so the alphabet has to use the four-element patterns, and 12 of those 16 became letters.
| Elements | Letters | Codes |
|---|---|---|
| 1 | E, T | . - |
| 2 | I, A, N, M | .. .- -. -- |
| 3 | S, U, R, W, D, K, G, O | ... ..- .-. .-- -.. -.- --. --- |
| 4 | H, V, F, L, P, J, B, X, C, Y, Z, Q | .... ...- ..-. .-.. .--. .--- -... -..- -.-. -.-- --.. --.- |
The real design decision was which letters got the 14 short slots. Look at the first three rows: alongside E and T, they hold many of the letters you meet on every line of English text, such as A, I, N, S and O. The bottom row holds Q, J, X and Z, which you can go a whole page without seeing.
Why short codes save time
Counting elements is only half the picture, because a dah lasts three times as long as a dit. Morse time is measured in units: a dit is 1 unit, a dah 3, the gap inside a letter 1, the gap between letters 3. Measured that way, here is what some letters cost to send:
| Letter | Code | Units |
|---|---|---|
| E | . |
1 |
| T | - |
3 |
| I | .. |
3 |
| A | .- |
5 |
| N | -. |
5 |
| S | ... |
5 |
| H | .... |
7 |
| M | -- |
7 |
| O | --- |
11 |
| C | -.-. |
11 |
| J | .--- |
13 |
| Q | --.- |
13 |
Now compare two four-letter words. NEAT, -. . .- -, is 14 units of letters plus three letter gaps of 3 units: 23 units. JAZZ, .--- .- --.. --.., is 40 units of letters plus the same 9 units of gaps: 49 units, more than twice as long. At 20 WPM that is 1.38 seconds against 2.94. Ordinary writing is full of words like NEAT and has very few like JAZZ, so giving the common letters the short codes shortens almost every message. On a busy telegraph line, time on the wire was the thing being saved.
The type case story
How did the code’s designers know which letters were common? The story that is usually told is that Alfred Vail, who worked with Samuel Morse on the telegraph, counted the type in a printer’s case. The idea behind it is that a printer stocks more pieces of the letters used most, so the counts in a type case would make a ready-made frequency table.
Treat it as a good story rather than settled history. It is often told, and the reasoning is sound, but how much of the code was Vail’s rather than Morse’s is not settled. Who invented Morse code goes into what is known about the two men and their work. However the letters were chosen, the idea is plain in the result: E and T, the two commonest letters in English, are the only one-element codes.
Where the code does not follow frequency
The fit is good, not perfect. A few places where it slips:
O is three dahs. O is a common letter in English, yet --- takes 11 units, longer than H or V. M, which turns up less often than O, is -- and takes 7.
Dahs are expensive. If you sort by element count, T, -, looks shorter than I, ... In time they are equal, 3 units each. M, --, takes exactly as long as H, ..... Counting elements hides that cost; counting units shows it.
Rarer letters sometimes beat commoner ones. V and K, which turn up less often in English than C and P, take 9 units each; C and P take 11.
The code we use is a revision. The Morse heard on radio today is International Morse, sometimes called Continental. It grew out of a revision usually credited to Friedrich Clemens Gerke in 1848, and it differs from American Morse, the landline code, which has spaces inside some characters (C, O, R, Y, Z and the ampersand). So those letters have different codes in the two versions: O’s three dahs, for one, are International, not American. Whatever the original reasoning, the code you learn today also carries that revision. American Morse vs International Morse sets the two side by side.
It was tuned for English. A language with a different mix of letters may get a looser fit from the same code.
Figures follow a different rule. Every figure is five elements, from 1, .----, through 5, ....., and 6, -...., through 0, -----. They do not follow frequency at all. The figures are regular instead, so you can work one out from its pattern, and most punctuation marks take five or six elements.
What this means when you learn
E and T are easy to miss. At 20 WPM an E lasts 60 milliseconds, a single blip between two letter gaps. It is easy to copy every T and still drop an E, because it is over before you notice it. Listen for the tick.
Short codes are the start of longer ones. E, ., is the beginning of I, S, H, A, R and many more. A receiver only knows a letter has ended when the 3-unit letter gap arrives, which is why spacing carries as much meaning as the sounds. Squeeze the gap in AN, .- -., and the receiver hears .--., which is P.
Common letters first gives you words sooner. Because E, T, A, N and I are both short and common, a learning order that starts with them lets you send real words almost at once. Which Morse code letters to learn first compares the usual orders.
Dot & Dash takes that route on a Mac: the first chapter of its course is E, T and A, so you key AT, ATE, EAT and TEA from the start, and HELLO is in reach by chapter 5. Its welcome teaches E as one dit and T as one dah, then has you send SOS.
Questions
Why is E a single dot in Morse code?
Because E is the most common letter in English. The code gives its shortest patterns to the letters used most, so E got one dit and T, also very common, got one dah.
Which Morse letters take longest to send?
J, Y and Q, at 13 units each, because each has three dahs. O, P, X, C and Z follow at 11 units.
Did Samuel Morse count letters in a printer’s type case?
The story is usually told about Alfred Vail, Morse’s partner, not Morse himself, and it is best treated as a story that is often told rather than settled fact. The code shows the idea clearly whoever applied it.
Is Morse code perfectly efficient?
No. O is long for such a common letter, V and K are quicker to send than the commoner C and P, and the figures were made regular rather than short. It is close to frequency order at the top and looser below.