The Code wheel is a
cryptographer's tool, easily made from two sheets of
construction paper. Write an evenly spaced
alphabet in a circle on one disc of paper (
this is harder than it sounds). Then, place that disc on another concentric, larger disc of paper and write on it another evenly spaced alphabet lined up with the one on the original sheet. Fasten the two so that the discs can spin with each other, and you have a basic code wheel.
The purpose of the code wheel is to make it easy to execute letter addition and subtraction. For example, if you have a code with key letter B (which replaces A with B, B with C, and so forth) you can generate a full key to the code by spinning the code wheel until B on one disc (the encrypted disc) lines up with A on the other (the clear text disc). Then you can encode or decode easily. Unfortunately, a single key letter code is very easy to crack with basic cryptanalysis skills.
Note that the ROT13 code is simply a key letter N code.
One of the nice parts about the code wheel is that it is easily set to a new code. If you have a message which is being sent in more than one key, this is vital.
A common way to send a message in more than one key is to rotate the key letter through a prearranged pattern (possibly a word without repeating letters, such as 'albedo'), called a code key word or code key phrase.
While these schema make it very difficult to break the code by hand, a computer can do a brute force solution in no time flat, provided it has been programmed to guess the kind of key you are using. If you want to beat a computer, using something so simple as a code key word is probably too simple. That does not mean that a lowly code wheel cannot be used in encoding and decoding a computer-proof code.
For example, you can use a One Time Pad, which is unbreakable in principle. If you are too lazy to send an extensive one time pad, you can arrange a code key phrase with your accomplice, which will be used only for a sufficiently short section of the code that it will also be unbreakable in principle, and use that short section to specify an accessible document to use as a less-than ideal one time pad (less than ideal because the pad itself will not be evenly distributed over the alphabet. A computer can solve this one too - the sum of two cleartext messages is easy to split apart for a machine).
Example: Alice and Bob, both being rennaissance lewd music fans, have arranged the key phrase "Fair Phyllis I saw sitting all alone" for the code-setting section of their code, which is to be 30 characters long exactly. Now, Alice chooses a document which she can be sure both she and Bob have identical copies of. Let's say she has picked the Hitchhiker's Guide to the Galaxy. So, she chooses an identifiable section of that to be her code document. She then adds the letters in "vogonpoemoddletters" to "fairphyllisisawsitti" using her code wheel, and then adds on an additional 11 nulls to make the message body start after the initial key phrase is done. Then, Alice proceeds to encode her message using the odd letters of the one vogon poem we are unlucky enough to be forced to read as the key. If Alice had a very very long message and wanted to be sure it could not be cracked, she could have specified a longer key document, so long as she could be sure that Bob's copy was identical to hers. For example, "MacbethactIsceneiiispokenonly" weighs in with one character free at the end.