A1Z26 cipher

Each letter becomes its number, with spaces left between words. Between letters switches the separator from hyphens to spaces or full stops. With spaces, a slash marks each word break instead.

Decoding is forgiving. Hyphens, spaces and full stops all work, so a string copied out of a puzzle decodes whichever way it was written. If only spaces sit between the numbers, a slash, a double space or a new line starts a new word. 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 and then Disney XD from 2012 to 2016. Every episode ended with a cryptogram over the credits.

The show rotated its ciphers on a schedule. The first season ran six episodes of Caesar shifts, seven of Atbash and then A1Z26, and the season finale stacked all three.

In a hunt, A1Z26 is the first thing to 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.

Ages 6 and up, about 15 minutes. Students write their name as numbers, add the numbers up, and compare totals: whose name scores highest? It mixes spelling with addition, and the word value calculator checks the sums.

Take it further: set a challenge to find a word worth exactly 50, or the highest-scoring word in a spelling list. Older students can try messages where the numbers are hidden in sums, such as 3 × 4 for L.