First a keyword is decided upon and written down, the remaining letters of the alphabet are then added to make sure that every letter of the alphabet has an encoded version. For example, using the keyword "secret" the rest of the alphabet will be laid out after the keyword reading "abdfghijklmnopquvwxyz". If the codeword contains a duplicate letter, then the second occurence of the letter in the codeword is ignored and the alphabet completed in the usual way:

Plaintext: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Encoded: S E C R T A B D F G H I J K L M N O P Q U V W X Y Z

Encoding the message "Purple Hell" using the keyword "secret" would give the code MUOMIT DTII.

The usual starting letter of the plaintext alphabet is A, occasionally, to make the cipher harder to crack, the starting letter of the alphabet is shifted. To decrypt we would then need to know the starting Keyletter as well as the Keyword. In the following example the keyletter has been shifted to P:

Plaintext: P Q R S T U V W X Y Z A B C D E F G H I J K L M N O
Encoded: S E C R T A B D F G H I J K L M N O P Q U V W X Y Z