Four square (idea)

The four square cipher is similar to the playfair cipher in its construction. Two '5x5' grids are created with mixed alphabets. These are placed at diagonal opposites of four squares. For the following example the keys of 'key number one' and 'number two key' shall be used.

               (1st cipher text)
(1st clear text)
  A B C D E       K E Y N U
  F G H I K       M B R O A
  L M N O P       C D F G H
  Q R S T U       I L P Q S
  V W X Y Z       T V W X Z

  N U M B E       A B C D E
  R T W O K       F G H I K
  Y A C D F       L M N O P
  G H I L P       Q R S T U
  Q S V X Z       V W X Y Z
(2nd cipher text)
                (2nd clear text)

The plain text is then split into groups of two letters. Nulls or some particular character may be added to make the pairings work. The message "how about dinner tonight" becomes:
HO WA BO UT DI NN ER TO NI GH TX

Next, the two letters are located in the clear text boxes. The original characters are in bold, the cypher in italic.

  . . . . . | . . . . .
  . . H ----------- O .
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . | . . | . . . | .
------|-----+-------|------
  . . | . . | . . . | .
  . . | . . | . . . | .
  . . C ----------- O .
  . . . . . | . . . . .
  . . . . . | . . . . .

The first plain text becomes the character in the first cipher text, and the second plain text becomes the character in the second cipher text. The fully enciphered stream is:
OC TU NA QL NO FC EL QL GW RT PX

Of course, this can also be extended to a 6x6 block with A-Z and 0-9 and 10x10 block to encode numeric data.