Why do some letters get one digit and others get two?

The eight most common letters sit on the top row and get a single digit each, which shortens the message. The remaining letters live on two lower rows and get two-digit codes that begin with one of the two blank-column digits.

What does straddling mean here?

It means the codes have mixed lengths but still decode unambiguously. No single-digit code is ever the first digit of a two-digit code, so when you read the digit stream you always know whether to take one digit or two.

What are the blank columns for?

The top row has two empty cells. The digits labelling those two columns become the prefixes for the two lower rows. Seeing one of those prefix digits in the ciphertext tells the decoder to read a two-digit group.

How does the keyword change the board?

The keyword builds a mixed alphabet by writing its unique letters first and then the rest of the alphabet. That mixed order is poured into the lower rows, so the same column labels with a different keyword produce a completely different cipher.

Where was this cipher actually used?

Straddling checkerboards were used by resistance groups and intelligence agents in the 20th century, and they form the substitution stage of larger systems such as the Nihilist and the Soviet VIC cipher.

Straddling Checkerboard Cipher

The straddling checkerboard is a fractionating substitution cipher: it turns each letter into one or two digits using a small keyed grid. Its hallmark is compression — the most frequent letters become single digits — combined with self-synchronising decoding, which is why it became the substitution layer of famous field ciphers like the Nihilist and the Soviet VIC.

How it works

The board is three rows of ten columns. The columns are labelled with a permutation of the digits 0–9.

Top row: the eight highest-frequency letters fill eight of the ten cells; two cells are left blank. Each of these letters is encoded by its single column digit.
Two blank columns: the digits labelling them become the prefix digits for the two lower rows.
Lower two rows: the rest of the mixed alphabet fills these. Each letter becomes a two-digit code — its row’s prefix digit followed by its column digit.

Decoding reads the digit stream left to right. If the next digit is one of the two prefix digits, take two digits; otherwise take one. Because no single-digit code equals a prefix, this is always unambiguous — the “straddling” property.

Example

With keyword CIPHER, columns 0123456789 and top row ASINTOER, the message ATTACK AT DAWN becomes:

0660202806240544

Here A is 0, T is 6, and the less common C is 20, K is 28, D is 24, W is 54, N is 4. Decoding the same digit string with the same board recovers the message exactly.

Notes

Only the 26 letters are placed on the board; spaces and punctuation are dropped, so group the output for transmission yourself.
For maximum compression, the top row should hold the genuinely most frequent letters of your language — for English, letters from ASINTOER are a common choice.
The board alone offers only weak security; its real strength comes when its digit output is further enciphered, as in the Nihilist and VIC ciphers.