A1Z26 cipher

Type on the left and the numbers appear on the right, one per letter, with spaces left between words. Between letters switches the separator.

Decoding is forgiving. Hyphens, spaces and full stops all work, so a string copied out of a puzzle decodes whichever way it was written. Numbers outside 1 to 26 are listed under the output rather than guessed at.

Each letter is replaced by its position in the alphabet. There's no key, so anyone who recognises the scheme can read it immediately. Calling it a cipher is generous.

It survives because it composes. Numbers can be added to, shifted or fed into another cipher, and a message in digits looks unlike text, which is enough to make a puzzle feel like a puzzle. Word boundaries stay visible, so letter counts leak: a run of three numbers at the start of a sentence is often "the".

The cipher is a fixture of puzzle hunts and alternate reality games, and the clearest recent example is Gravity Falls, which ran on Disney Channel from 2012 to 2016. Every episode ended with a cryptogram over the credits.

The show rotated its ciphers on a schedule. The first season cycled through Caesar shifts, Atbash and A1Z26, and later episodes combined them. Fans decoded each one within hours of broadcast, which was the point: creator Alex Hirsch has said the puzzles were there to reward the people who looked.

That's the honest place for A1Z26. It's a first step into a hunt, the thing you try when a page of numbers appears and none of them go past 26.

The Polybius square also turns letters into digits, but as grid coordinates, so nothing goes above 5. Tap code does the same over sound.

For the numbering computers actually use, where A is 65 and a is 97, see ASCII.