Sue de Coq
Sue de Coq is an advanced, elegant technique that makes eliminations in two directions at once — along a box and a line — from a small cluster of cells where they cross. It's named after the forum handle of the solver who first described it (originally under the drier title "Two-Sector Disjoint Subsets").
The idea
Take two or three cells sitting in the intersection of a box and a line (a row or column). Suppose those cells, between them, carry a set of candidates that can be neatly divided so that some digits must come from the box's remaining cells and others from the line's remaining cells — with the intersection cells acting as a shared pool. When the arithmetic of candidates lines up just right, you can eliminate digits from both the rest of the box and the rest of the line simultaneously.
In modern terms, a Sue de Coq is a compact case of the Almost Locked Set (ALS) family: the intersection cells plus one ALS in the box and one in the line combine to lock digits down. You don't need the full ALS theory to use it — the pattern is recognisable on its own — but that's where it comes from.
The intersection cells r1c1{1,2} and r1c2{3,4} (amber) pool with a box ALS r3c1{1,3} (blue) and a line ALS r1c8{2,4} (green). Together the four cells lock {1,2,3,4}: 1 and 3 must stay in box 1, and 2 and 4 in row 1 — so they clear from the rest of the box and the rest of the row at once.
Step by step
- Look at a box-line intersection — the two or three cells a box shares with a row or column.
- Count the candidates there. You want a set of digits large enough that some are forced into the box and the rest into the line.
- Match the almost-locked sets in the box's remainder and the line's remainder.
- Eliminate the appropriate digits from both the rest of the box and the rest of the line.
Is it worth learning?
Sue de Coq is firmly expert territory — powerful, but not something you'll reach for on an everyday grid. It rewards solvers who enjoy the almost-locked-set way of thinking, and it can crack positions where wings and fish stall. If it appeals, it's a natural doorway into the wider ALS techniques.
Common mistakes
- Wrong candidate count. The intersection cells must carry exactly enough digits to split cleanly between box and line.
- Overlapping sets. The box ALS and line ALS must be disjoint apart from the shared intersection.
Which Sudoku types is this best for?
The same logic applies across variants, but it pays off more in some than others.