The Sue de Coq Sudoku Technique Explained

Start at a box-line intersection

Choose the three cells where one row crosses one 3 by 3 box, or use the matching column version. Focus on the unsolved cells in that intersection and collect all candidates appearing there. The classic small pattern often uses two intersection cells containing four candidates in total, such as heart, circle, crescent, and lotus. Those four candidates must fill the two intersection cells, but the useful step is showing that they can be split into two locked groups. One group will be controlled by the rest of the row outside the box, and the other by the rest of the box outside the row. The labels can vary, and the intersection can contain more than two cells in broader forms. The core count is what matters: the intersection cells contain a tight candidate set, and the outside cells demonstrate how that set must divide. In Shapedoku, full Notes make the pattern much easier to verify on Extreme boards.

Separate the line candidates from the box candidates

Imagine the two intersection cells use candidates A, B, C, and D. Elsewhere in the same row, suppose one bivalue cell contains A and B. Elsewhere in the same box, suppose one bivalue cell contains C and D. The row cell must take one of A or B, and the box cell must take one of C or D. That leaves the two intersection cells to take one candidate from each group. The result is two overlapping locked sets. Candidates A and B are confined to the intersection cells plus the bivalue row cell, all of which lie in the same row. Candidates C and D are confined to the intersection cells plus the bivalue box cell, all of which lie in the same box. You may therefore remove A and B from every other cell in that row, and remove C and D from every other cell in that box. Real Sue de Coq patterns can be larger, but the basic version is the safest place to learn the logic.

Verify the pattern with counts, not appearance

Sue de Coq is easy to misread because several nearby pairs can look persuasive. Count the intersection cells, count their combined candidates, and identify the exact outside sets that control each candidate group. The line group and box group must be disjoint in the basic form. If one candidate appears in both groups, the simple elimination no longer follows. Also confirm that every cell used by the row set lies in the same row and outside the box, while every cell used by the box set lies in the same box and outside the row. Do not include a convenient cell merely because it sees part of the pattern. Once the structure is valid, mark only the eliminations justified by the corresponding group. Then rescan the affected row and box for singles or smaller subsets. A Sue de Coq rarely solves a puzzle by itself. Its value is compressing a messy intersection into one clean candidate removal.

1. Choose a row-box or column-box intersection with two unsolved cells containing four candidates in total.
2. Find one bivalue cell on the line outside the box using two of those candidates.
3. Find one bivalue cell in the box outside the line using the other two candidates.
4. Confirm that the line pair and box pair are disjoint in the basic form.
5. Eliminate the line-pair candidates from the rest of the line.
6. Eliminate the box-pair candidates from the rest of the box.