The Alphabet-Cipher
Lewis Carroll
1868

     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
    
  A  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 
  B  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 a  
  C  c d e f g h i j k l m n o p q r s t u v w x y z a b 
  D  d e f g h i j k l m n o p q r s t u v w x y z a b c  
  E  e f g h i j k l m n o p q r s t u v w x y z a b c d 
  F  f g h i j k l m n o p q r s t u v w x y z a b c d e 
  G  g h i j k l m n o p q r s t u v w x y z a b c d e f 
  H  h i j k l m n o p q r s t u v w x y z a b c d e f g 
  I  i j k l m n o p q r s t u v w x y z a b c d e f g h 
  J  j k l m n o p q r s t u v w x y z a b c d e f g h i 
  K  k l m n o p q r s t u v w x y z a b c d e f g h i j 
  L  l m n o p q r s t u v w x y z a b c d e f g h i j k 
  M  m n o p q r s t u v w x y z a b c d e f g h i j k l 
  N  n o p q r s t u v w x y z a b c d e f g h i j k l m 
  O  o 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 
  P  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 
  Q  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 p 
  R  r s t u v w x y z a b c d e f g h i j k l m n o p q 
  S  s t u v w x y z a b c d e f g h i j k l m n o p q r 
  T  t u v w x y z a b c d e f g h i j k l m n o p q r s 
  U  u v w x y z a b c d e f g h i j k l m n o p q r s t 
  V  v w x y z a b c d e f g h i j k l m n o p q r s t u 
  W  w x y z a b c d e f g h i j k l m n o p q r s t u v 
  X  x y z a b c d e f g h i j k l m n o p q r s t u v w 
  Y  y z a b c d e f g h i j k l m n o p q r s t u v w x 
  Z  z 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 
    
  
Explanation

EACH column of this table forms a dictionary of symbols representing the
alphabet: thus, in the A column, the symbol is the same as the letter
represented; in the B column, A is represented by B, B by C, and so on.

To use the table, some word or sentence should be agreed on by two
correspondents. This may be called the `key-word', or `key-sentence',
and should be carried in the memory only.

In sending a message, write the key-word over it, letter for letter,
repeating it as often as may be necessary: the letters of the key-word
will indicate which column is to be used in translating each letter of
the message, the symbols for which should be written underneath: then
copy out the symbols only, and destroy the first paper. It will now be
impossible for any one, ignorant of the key-word, to decipher the message,
even with the help of the table.

For example, let the key-word be vigilance, and the message `meet me on
Tuesday evening at seven', the first paper will read as follows --

v i g i l a n c e v i g i l a n c e v i g i l a n c e v i
m e e t m e o n t u e s d a y e v e n i n g a t s e v e n
h m k b x e b p x p m y l l y r x i i q t o l t f g z z v

The second will contain only 'h m k b x e b p x p m y l l y r x i i q t o l t
 f g z z v'.

The receiver of the message can, by the same process, re-translate it
into English.

N.B. -- If this table be lost, it can easily be written out from memory,
by observing that the first symbol in each column is the same as the
letter naming the column, and that they are continued downwards in
alphabetical order. Of course it would only be necessary to write out the
particular columns required by the key-word: such a paper, however, should
not be preserved, as it would afford means for discovering the key-word.

CLARIFICATIONS: For the example given we will explain more precisely how to 
encrypt/decrypt: 

To encrypt, line the plaintext up with the keyword "vigilance" as indicated below:

v i g i l a n c e v i g i l a n c e v i g i l a n c e v i
m e e t m e o n t u e s d a y e v e n i n g a t s e v e n

Now to encrypt the first letter in our message, find the letter "V" in the alphabet above 
the block of lowercased letters.  Next, find the letter "M" in the alphabet to the left of 
the main block of letters.  Now, find where the column that begins with "V" intersects the
row that begins with "M".  This is exactly on the letter "h".  The letter "h" is the first
letter of our ciphertext.  

To decrypt, line the ciphertext up with the keyword "vigilance" as indicated below:

v i g i l a n c e v i g i l a n c e v i g i l a n c e v i
h m k b x e b p x p m y l l y r x i i q t o l t f g z z v

Now to decrypt the first letter in our message, find the letter "V" in the alphabet above 
the block of lowercased letters.  Follow the letters down that column until you run into
the letter "h".  Now, follow that row left until you hit the letter "M".  So, "M" is the
first letter in your decrypted message.