Tabula recta

In cryptography, the tabula recta is a square table of alphabets, each row of which is made by shifting the previous one to the left. The term was invented by Johannes Trithemius in 1518.

Trithemius used the "tabula recta" to define a polyalphabetic cipher which was equivalent to Leon Battista Alberti's cipher disk except that the alphabets are not mixed. The "tabula recta" is often referred to in discussing pre-computer ciphers, including the Vigenère cipher and Blaise de Vigenère's less well-known (but much stronger) autokey cipher. All polyalphabetic ciphers based on Caesar ciphers can be described in terms of the "tabula recta".

In order to encrypt a plaintext, one locates the row with the first letter to be encrypted, and the column with the first letter of the key. The letter where the line and column cross is the ciphertext letter.