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Sue de CoqSue de Coq sudoku technique
Sue de Coq lives in the overlap between a line and a box — those three shared cells. When two of them jointly hold candidates that split cleanly into a set completed within the line and a set completed within the box, both digit sets get locked simultaneously. The line loses its set outside the overlap, and the box loses its set outside the overlap, in one move.
How to find a sue de coq
- Examine the two- or three-cell intersection of a row or column with a box.
- Check whether its candidates divide into digits that must finish on the line and digits that must finish in the box.
- Remove the line's digits from the rest of the line, and the box's digits from the rest of the box.
- Treat it as a doubled locked set — that is all the pattern really is.
The pattern on the grid
A real position — every pencil mark computed, every red candidate a verified elimination. Toggle to see the grid after the move.
Sue de Coq: r2c5,r3c5 {2468} + r9c5 (line), r3c6 (box) => r4c5<>8, r6c5<>8
745319862196700433328500977037200159952173648061905723613597284289431576574600391
Standard 81-character format (0 = empty) — paste it into your solver of choice and verify the move yourself.
Worked example
The two overlap cells at r2c5, r3c5, r3c6, r9c5 split their combined candidate set between a line completion and a box completion (the exact split is in the notation below). Both directions lock at once, producing the double elimination r4c5<>8, r6c5<>8 — one move, two units swept.
When it fails
The candidate split must be airtight — every digit assignable to exactly one side (row, box, or locked in the overlap). If a digit could complete on either side, the pattern proves nothing and the 'eliminations' are guesses.