Example
An example key is:
EPSDUCVWYM.ZLKXNBTFGORIJHAQ
We then make three squares from this:
square 1 square 2 square 3
1 2 3 1 2 3 1 2 3
1 E P S 1 M . Z 1 F G O
2 D U C 2 L K X 2 R I J
3 V W Y 3 N B T 3 H A Q
If we take a plain text message of "THIS IS A TEST", we locate the text in the squares defined above:
THIS IS A TEST -------------- T - 233 H - 331 I - 322 S - 113 I - 322 S - 113 A - 332 T - 233 E - 111 S - 113 T - 233
Next we would order as:
THISISATEST ----------- 23333132211 33221133322 33111113233
And we would read the code in a horizontal way to give:
233 333 321 321 311 111 331 233 232 123 123
And then substitute back the letters on the grid:
233 333 321 321 311 111 331 233 232 123 123 T Q R R F E H T B C C