Extended Unique Rectangle Sudoku: The Six-Cell Deadly Pattern

What the extended unique rectangle sudoku pattern looks like

The extended unique rectangle sudoku pattern uses six cells instead of four and three candidate digits instead of two. Put the six cells in three columns of a single stack and two rows from different bands, for example r1c1, r1c2, r1c3, r4c1, r4c2 and r4c3, which together cover box 1 and box 4. Let the three shared digits be 2, 4 and 6. If all six cells held only 2, 4 and 6, the grid would carry a hidden freedom: you could permute those three digits between the two rows and produce a second complete solution. A proper puzzle allows no such freedom, so the pattern must be broken, and that break becomes your elimination.

Why the deadly pattern cannot survive

The logic mirrors the ordinary rectangle. In any solution, row r1 places 2, 4 and 6 across c1, c2 and c3 in some order, and row r4 places the same three digits in the remaining order. Because both rows draw from the exact same three values in the exact same three columns, the two arrangements can be swapped wholesale without disturbing any row, column or box. That swap would be a second solution. Since a valid Sudoku has only one, at least one of the six cells must carry a value outside the trio of 2, 4 and 6, and that requirement is precisely what lets you remove candidates.

A worked six-cell elimination

Suppose five of the cells are bare, reading exactly 2, 4 and 6, while r4c3 reads 2, 4, 6 and 8. The five bare cells cannot break the deadly pattern on their own, so the whole burden falls on r4c3. It must take the value that lies outside the trio, which means r4c3 is forced to give up 2, 4 and 6 and settle as 8. In notes terms you erase 2, 4 and 6 from r4c3, collapsing it to a naked single. Had two cells carried extras instead of one, you could not pin a single value, yet you could still remove any candidate combination that would let all six fall back into the pure trio trap.

• Look for three columns in one stack, or three rows in one band, sharing the same three candidates.
• Check that the six cells sit in exactly two boxes, not three.
• Confirm every cell contains all three trio digits, with extras allowed.
• Count the cells that carry extras; one extra cell behaves like rectangle Type 1.
• Erase the trio from the odd cell, or clear any candidate that would restore the deadly set.

Bringing it to Shapedoku

A six-cell pattern is a lot to hold in your head, and clean notes make the difference. In Shapedoku the three shapes glow in three distinct colors, so an extended unique rectangle sudoku figure reads almost like a small colored grid inside the board. Notes with smart cleanup keeps the six cells honest by trimming candidates that can no longer be true, which makes the odd cell with an extra shape stand out at once. If you are not sure the figure is genuine, the free solver on shapedoku.com confirms the geometry and shows the removal, and Extreme mode at app.shapedoku.com is where these larger deadly patterns actually appear. Learn the four-cell rectangle first, then this six-cell cousin will feel like a natural next step.