Here's a type of cryptogram I call a SKIPtogram. It is based on the skip distances between two consecutive letters in the plaintext, and is enciphered and deciphered using a simple device made with two strip of graph paper.

The first strip of graph paper is three cells high and 26 cells wide.  The second strip of graph paper is one cell high and 39 cells wide as shown in figure 1. I find it handy to glue the strips to a piece of light cardboard to keep them from flopping around.

Figure 1. The Skiptogram ruler.

The first strip contains two alphabets, the top one in normal alphabetical order and the bottom one in mixed order based on a keyword, "ROAST BEEF" in this example. The second strip contains one full alphabet in normal alphabetical order plus a repeat of the first half of the alphabet, also in alphabetical order.  The second strip is placed between the two rows of the larger strip, so that it functions like the slider of a slide rule. We will refer to the larger strip as the "body" of the slide rule and the narrow slip we will call the "slider".

It helps to write the alphabets in distinctive colors to help keep them straight.  To encipher a cleartext message choose a key letter and tack that key letter on to the start of each word. For example, using 'q' as the key letter:

cleartext: Once upon a midnight dreary with key letter: qonce qupon qa qmidnight qdreary

Now to encipher the first letter of the message we look at the first two letters, q and o.  Move the slider, until the upper letter 'q' on the body is directly over the letter 'o' on the slider as shown in figure 2.

Figure 2. 'q' positioned over 'o'.

Now find 'A' on the slider and read off the ciphertext letter from the bottom row of the body, in this case, the letter 'A'. This will be the first ciphertext letter.

The next cipher text letter is found by taking the next pair of cleartext letters, 'o' and 'n'. Notice that the 'o' gets used again, this time as the first letter of the pair. Moving the slider so that 'o' is over 'n' we find 'A' on the slider and read off the next cipher text letter below it on the body, which as shown in figure 3, is 'O'.

Figure 3. 'o' positioned over 'n' to find ciphertext 'O'.

The next pair of cleartext letters, 'n' and 'c', yeilds ciphertext 'H', as shown in figure 4.

Figure 4. 'n' over 'c' yeilds ciphertext 'H'.

Finally, the last pair in the word, 'c' and 'e' yeilds 'Y' under the rightmost 'A' on the slider.

Figure 5. 'c' over 'e' yeilds ciphertext 'Y'.

So the cleartext 'once' becomes ciphertext 'AOHY'. Notice that in spite of adding an extra letter on the beginning of each word the completed ciphertext for that word is the same length as the original. That's because a four letter word only has three pairs of consecutive letters.  Also, using the same key letter added to the head of all words gives the recipient of the message a starting point for the process of decrypting the word. Without the key letter the enciphered word would be one letter shorter than the cleartext word and could be deciphered in 26 different ways. The use of the key letter eliminates that ambiguity in the message.

Here's the complete message as enciphered using the skiptogram slide rule:

once upon a midnight dreary AOHY WBOO M TTBMBAZO JIJTDQ

One desirable trait of hand-encryption methods is that human error does not destroy the whole message. In this case, a single letter miscopied can disrupt the decipherment of the rest of the word, however the garbled word is easily reconstructed by recognizing that in becoming garbled the word was merely shifted some fixed distance down the alphabet. If the decoded ciphertext yeilds "hovtf" we can look for simple shifts and discover that "-vtf" shifted one alphabet letter to the left is "-use" yeilding "house".

Self-correction could be added to the cipher by placing an additional key letter on the rightmost end of the word. This would increase the length of each word by one letter. Using Q as the key letter, 'once" from the message above is enciphered from 'quonceq' and would become AOHYC, with the 'C' representing the final 'eq' letter pair. Now if the word does not decipher properly due to an error the recipient can decipher the word from right to left instead of left to right yeilding all the correct letters except for the single letter that was in error.

Deciphering the message is done by positioning the slider so that the 'A' on the slider is above the first cipher text letter on the body. Since we know the key letter is 'q' we read off the first cleartext letter as whatever falls beneath 'Q' on the upper scale of the body. Figure 6 shows the process of recovering the first cleartext letter 'o' from the first ciphertext letter A.

Figure 7. Deciphering the first letter 'A' by finding 'Q' on the top body row and reading 'o' beneath it.

Notice that figure 7 is identical with figure 2 because the process of deciphering is identical with the process of ciphering except that we start with the known ciphertext letter on the bottom, jump to the top to find 'Q' and read down to find the cleartext letter.

To decipher the next letter, in place of 'Q' we use the letter just deciphered, 'o' to locate the cleartext letter.  Again, place the slider 'A' over the ciphertext letter 'O', find the previous cleartext letter 'o' on the top body row and read down to 'n', which is the next cleartext letter.

Figure 8, which is identical to figure 3.

In this way each cleartext letter is deciphered by using the just deciphered letter that precedes it as the key.

Using the Slide Rule to do Keyed Polyalphabetics like the Vigenere

Instead of using adjacent letters the top row could be the next letter from a keyword and the slider could be the cleartext letter.  For example, enciphering the phrase above using the key "DOG" would proceed as follows: Pair off the cleartext letters with the key letters.

DOGD OGDO G DOGDOGDO GDOGDO once upon a midnight dreary

1. Locate the first keyword letter 'D' on the top row and place the first cleartext letter 'o' under it.
2. Readout the first ciphertext letter 'L' as shown in figure 9.
3. Find the second ciphertext letter by placing 'O' over 'n' which points to 'O' on the bottom row.
4. Find the third ciphertext letter by placing 'G' over 'c' which points to 'T' on the bottom row.
5. and so on.

Figure 9. Using the slide rule to do a key polyalphbetic. 'D' over 'o' yeilds 'L'.

DOGD OGDO G DOGDOGDO GDOGDO once upon a midnight dreary LOTZ WNLO E NUSMKRWV SIGEIM <- The complete polyalphabetic ciphertext