Paste A's and B's (or any two symbols) to decode, or read a message hidden in capitals.
Francis Bacon described it in 1605. Every letter becomes five symbols from a two-letter alphabet. With A and B: A is AAAAA, B is AAAAB, C is AAABA, up to Z. It's a five-bit binary code three centuries before computers, with A for 0 and B for 1.
The trick was never the code itself. It was where Bacon hid it. Any two kinds of anything can carry the A's and B's: two typefaces, roman and italic, capital and small letters, even two kinds of flower in a picture. The message rides inside a perfectly innocent text, which makes this steganography, hiding that a message exists at all.
Bacon's alphabet had 24 letters: I and J shared a code, and so did U and V, as they did in the Latin of his day. The modern version gives every letter its own code, which shifts every letter after I by one. If a decode is close but slightly off from J onward, switch the alphabet setting.
Each letter becomes a group of five A's and B's: A is AAAAA, B is AAAAB, C is AAABA, and so on. That's binary with A for 0 and B for 1.
Francis Bacon's original merged I with J and U with V, so it has 24 codes. The modern version gives every letter its own. If a decode is nearly right but a few letters are off by one, switch versions.
Bacon meant it to be. Write any innocent sentence and use two styles of letter: say, lowercase for A and capitals for B, or two typefaces. Choose “Letter case” to read capitals as B.
Paste it as it is. Any text with exactly two different symbols is read as A and B. If it comes out as nonsense, tick “Swap A and B”.