In the Vigenère cipher, the first row of the tableau is filled out with a copy of the plaintext alphabet, and successive rows are simply shifted one place to the left. (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. Each letter of the keyword is used in turn, and then they are repeated again from the beginning. So if the keyword is 'CAT', the first letter of plaintext is enciphered under alphabet 'C', the second under 'A', the third under 'T', the fourth under 'C' again, and so on, or if the keyword is 'RISE', the first letter of plaintext is enciphered under alphabet 'R', the second under 'I', the third under 'S', the fourth under 'E', and so on. In practice, Vigenère keys were often phrases several words long.
In the Vigenère cipher, the first row of the tableau is filled out with a copy of the plaintext alphabet and successive rows are simply shifted one place to the left. (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. Each letter of the keyword is used in turn, and then they are repeated again from the beginning. So if the keyword is 'CAT', the first letter of plaintext is enciphered under alphabet 'C', the second under 'A', the third under 'T', the fourth under 'C' again, and so on, or if the keyword is 'RISE', the first letter of plaintext is enciphered under alphabet 'R', the second under 'I', the third under 'S', the fourth under 'E', and so on. In practice, Vigenère keys were often phrases several words long.