Different alphabets for different parts of the ciphertext. Flattens the frequency distribution and is not vulnerable to Frequency Analysis.
Example
plaintext: H E L L O W O R L D
alphabet: 1 2 3 4 5 1 2 3 4 5
Different alphabets for different parts of the ciphertext. Flattens the frequency distribution and is not vulnerable to Frequency Analysis.
plaintext: H E L L O W O R L D
alphabet: 1 2 3 4 5 1 2 3 4 5