Sue de Coq

Sue de Coq is the technique named after the Sudoku player who first published the pattern in the early 2000s. It's a niche but distinctive move: at the intersection of a row (or column) and a box, a small group of cells holds candidates that, viewed from two perspectives, behave as two Almost Locked Sets layered on top of each other. The constraint forces eliminations both inside the row and inside the box.

The shape

Pick two or three cells at the intersection of a row and a box — the cells that belong to both that row and that box. Look at the candidates in those cells. Sue de Coq fires when the candidates split cleanly into two groups: one group of candidates that must come entirely from the row's cells outside the box, and one group that must come entirely from the box's cells outside the row.

The classic two-cell version. Suppose two cells in the row-box intersection together hold candidates {a, b, c, d}. Suppose further that the row outside this box has cells whose candidates form an Almost Locked Set on {a, b} — meaning the digits a and b in this row must come from those external cells. Suppose the box outside this row has cells whose candidates form an Almost Locked Set on {c, d}. The intersection cells then must hold one of {a, b} and one of {c, d}, with the partition forced by the two ALSes.

The eliminations: a and b can be removed from any other cell of the row outside the intersection that's in the row's ALS but isn't part of it. c and d can be removed similarly from the box's ALS region.

Why it's named, not subsumed

Many advanced patterns are special cases of ALS-XZ or AIC, and a fluent ALS solver can produce Sue de Coq's eliminations without naming them. Sue de Coq earns its own label because the visual pattern — the row-box intersection with the cleanly partitioned candidate split — is recognisable on the grid in a way that the more general ALS reasoning isn't.

In other words: ALS-XZ tells you what's true; Sue de Coq tells you what to look for. The pattern is a useful entry point into ALS-style reasoning for solvers who think geometrically rather than algebraically.

When you'll see it

Sue de Coq turns up surprisingly often once the eye is trained for it — most expert puzzles have at least one row-box or column-box intersection where the candidate distribution permits the move. The cost is the diagnosis: counting candidates across two regions and checking the partition takes some discipline. Solvers who push past simple ALS-XZ into Sue de Coq tend to do so because the pattern is geometrically distinctive and easier to spot once internalised.