Francis Bacon's biliteral cipher (1605) encodes each letter of the alphabet as a 5-character sequence of two symbols — traditionally A and B. It's essentially a 5-bit binary code, making it one of the earliest examples of binary encoding for text. Bacon used it for steganography: by hiding the two forms in different typefaces of an innocent cover text, a secret message could be concealed in plain sight.
Pick any cover text — every 5 letters conceal one secret letter. Bold = B, normal = A.