TheCodeBreakers
First delete all vowels, or as much of the message as possible without
running the risk of multiple reconstructions, and then encipher the
residue." Experts who have attacked cryptograms from whose plaintexts
only the letter e has been eliminated have found that the difficulty of
solution increased noticeably. Reducing redundancy is especially
effective because it robs the cryptanalyst of one of his chief tools for
attack instead of just bolstering the wall of secrecy. Cryptographers of
the Italian Renaissance
did this when they ordered cipher clerks to drop the second letter of a
doublet, as the second / in sigillo.
Such techniques rely upon the cipher clerks' knowledge of their
language to supply the suppressed elements of redundancy.
Abbreviations likewise may have such low redundancy, may require such
an extensive furnishing of information, as bn for battalion, that they may
annaabi.ee 
