The rhythm lab

Morse code is not heard as dots and dashes but as rhythm. From that follows directly which characters get confused: not the ones that look alike, but the ones that sound alike.

And that can be computed. Two characters are neighbors when they differ by exactly one step - one element more, one less, or one flipped. This page works the neighbourhoods out from the character table instead of keeping a list. Among the 26 letters there are 87 such pairs.

Why only two answers

With twenty-six buttons you measure whether someone knows a character. With two you measure whether they hear the difference - and that is what S/H or U/V actually turn on.

The price is that blind clicking already scores fifty per cent. So the result reported is not the hit rate but how far it sits above guessing: 50 per cent is zero, 100 per cent is one, and 75 per cent is half a point. Without that conversion, blind clicking would look like half mastery.

The classic pairs

A few turn up for almost everyone, and they all have the same cause: S and H differ only in the number of dots, three against four. U and V likewise, D and B too. Swapping them means you are still counting instead of hearing.

The remedy is not to repeat one character but to hear both in alternation until the difference is a sound shape rather than a sum. That is exactly what this drill does - and the coach assembles the same pairs from your training history.

The second kind are the flipped ones: E and T, A and N - no, A and N precisely not. They are mirrored, which is two steps, and so they sound less alike than the written form suggests. The map below shows which characters really are neighbors.

A map of the code

Below, the alphabet is ordered by number of elements: a level with two characters, then four, then eight, then twelve. That is not chance but the construction - each level doubles the possibilities, because a dot or a dash fits onto every pattern.

That E and T sit at the top and Q and Y at the bottom has a practical reason: in 1837 Alfred Vail counted the type in a newspaper's cases and gave the commonest letters the shortest signals. More under history, and anyone wanting to see the structure as a tree will find it in the Morse tree.