If BAT is coded as YZG, the rule is that each letter is replaced by

  • A. the letter three places ahead
  • B. the letter three places behind
  • C. its opposite letter in the alphabet, A for Z and B for Y
  • D. the next vowel

Explanation

B pairs with Y, A with Z and T with G, each letter being replaced by its mirror image counting from the other end of the alphabet. The check is that the positions of the pair always add to 27. This atbash style code appears regularly and is easy to recognise once the sum rule is known.

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