The Zodiac's Z340 cipher — 340 symbols arranged in a 17-by-20 grid — defeated professional and amateur cryptographers for fifty-one years. When an international team finally cracked it in December 2020, they found it used two layered techniques not present in the simpler Z408. The first was the same homophonic substitution used in Z408, where multiple symbols map to the same letter. The second was a structural trick that had misdirected decades of solvers. What was the second technique, and why did it confuse cryptanalysts for so long?