What is tabula recta?

Tabula Recta

In cryptography, the tabula recta (from Latin tabula rēcta) 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 1508, and used in his cipher.

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Tabula Recta
... In cryptography, the tabula recta (from Latin tabula rēcta) is a square table of alphabets, each row of which is made by shifting the previous one to the left ... 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 autokey cipher ...
Substitution Cipher - Polyalphabetic Substitution
... (Such a simple tableau is called a tabula recta, and mathematically corresponds to adding the plaintext and key letters, modulo 26.) A keyword is then used to choose which ciphertext alphabet to use ... words, but simplified by the fact that usually a tabula recta had been employed ... the same as the Vigenère, except the tabula recta is replaced by a backwards one, mathematically equivalent to ciphertext = key - plaintext ...