Straddling Checkerboard

Some articles on straddling checkerboard, checkerboard:

VIC Cipher - History
... In both versions, the plaintext was first converted to digits by use of a straddling checkerboard rather than a Polybius square ... and memorized the 8 most frequent letters of English (to fill the top row of the checkerboard) through the mnemonic (and slightly menacing) phrase "a sin to err ... The standard English straddling checkerboard has 28 character slots and in this cipher the extra two became "full stop" and "numbers shift" ...
Nihilist Cipher - Later Variants or Derivatives
... In both versions, the plaintext was first converted to digits by use of a straddling checkerboard rather than a Polybius square ... the 8 most frequent letters of English (to fill the top row of the checkerboard) through the mnemonic (and slightly menacing) phrase "a sin to err" (dropping the second "r") ... The standard English straddling checkerboard has 28 characters and in this cipher these became "full stop" and "numbers shift" ...