secret are still present in ETCRSE—but they lose their positions, while in substitution the letters retain their positions but lose their identities. Transposition and substitution may be combined. Substitution systems are much more diverse and important than transposition systems. They rest on the concept of the cipher alphabet. This is the list of equivalents used to transform the plaintext into the secret form. A sample cipher alphabet might be: plaintext letters abcdefghijklm cipher letters LBQACSRDTOFVM plaintext letters nopqrstuvwxyz cipher letters HWIJXGKYUNZEP This graphically indicates that the letters of the plaintext are to be replaced by the cipher letters beneath them, and vice versa. Thus, enemy would become CHCME, and swc would reduce to foe. A set of such correspondences is still called a "cipher alphabet" if the plaintext letters are in mixed order, or even if they are missing, because cipher letters always imply plaintext letters.
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