Letters become row-and-column numbers. Add a keyword to scramble the square.
Write the alphabet into a 5×5 grid. Twenty-six letters don't fit in twenty-five cells, so I and J share. Number the rows and columns 1 to 5. Every letter becomes two digits, its row and then its column: H is in row 2, column 3, so it's 23. TREASURE becomes 44 42 15 11 43 45 42 15.
The Greek historian Polybius described it around 150 BC for signalling with torches: hold up so many in the left hand for the row, so many in the right for the column. It's the ancestor of a long line of ciphers. Tap code is the same idea knocked on a wall, and Playfair, Bifid and ADFGVX all start from a square like this one.
Fill the square with a keyword first and the rest of the alphabet after it, and the numbers no longer line up with the plain alphabet. Without the keyword, the code becomes a substitution cipher on pairs of digits. To crack one, turn each number pair into any letter consistently and run the result through the substitution solver.
Only the digits 1 to 5, usually in pairs. No pair goes above 55 and no digit is 0 or above 5. The cipher identifier checks for exactly that.
Write the alphabet into a 5×5 grid, with I and J sharing a cell. Each letter becomes two digits: its row, then its column. H is in row 2, column 3, so it becomes 23.
It fills the square first, like Playfair: the keyword's letters go in, then the rest of the alphabet. Without the keyword, the numbers don't map back to the right letters.
Tap code is a Polybius square with C and K sharing a cell instead of I and J, and the numbers are knocked rather than written. It has its own tool.