TheCodeBreakers
unambiguously separate them when they are run together. The
cryptanalyst, however, not knowing which digits are singletons and
which form,pairs, may divide the ciphertext incorrectly, thereby
"straddling" many of the true pairs and combining two singletons into a
false
pair. The device also reduces the length of the numerical text as
compared with checkerboards in which all letters are replaced by
numerical pairs. Straddling was first employed by the Argentis in some of
their 16th-century papal ciphers (one wonders whether the atheistic
Communists knew!).
The straddling checkerboard produces single-digit equivalents by
leaving the side coordinate off one of the rows of the checkerboard. A
letter in this row is enciphered by just the single coordinate above it. If
ambiguity is to be avoided, none of these singletons can start a two-digit
group. Hence none caa be used as a side coordinate (which is read first).
annaabi.ee 
