Sue de Coq in Sudoku: How It Works and Whether You Need It

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Published April 16, 2026 · Updated October 14, 2026

Twenty-six digits are given in puzzle 81 of our Evil sudoku bank. After 24 naked and hidden singles and one W-Wing, 31 cells are empty, no single is left, and an Almost Locked Set is the one move any of our Sudoku Helper’s eighteen techniques can find. Look instead at the spot where column 9 runs through box 6. Two cells there are empty, R5C9 and R6C9, and between them they hold four digits: 2, 3, 5 and 8. Further down the column, R9C9 can only be 2 or 8. Inside the box, R5C7 can only be 3 or 5.

Those four cells must hold 2, 3, 5 and 8, one each. So column 9 gets its 2 and its 8 from them, and box 6 its 3 and its 5. The 8 goes from R1C9 and the 5s from R4C7 and R4C8, and R1C8 is left as the only place for a 5 in column 8. That is a Sue de Coq.

It is one of the prettiest deductions in sudoku, and on our puzzles it is hardly ever the only way on. We added it to our helper’s techniques and ran them over the first 500 Evil puzzles. It moved 45 times, and every time the helper had a move of its own ready; at the helper’s own dead ends a Sue de Coq turned up 5 times in 168. The player who first posted it, in 2005, said much the same.

What is a Sue de Coq in sudoku?

A Sue de Coq, also called Two-Sector Disjoint Subsets, works where a line (a row or a column) crosses a box. It needs three things:

1. A crossing with two digits too many: two empty cells where the line meets the box, holding four candidates between them, or three holding five.
2. A line partner: a cell further along the line, outside the box, holding two of those digits and nothing else.
3. A box partner: a cell elsewhere in the box, off the line, holding two of the others. The two partners share no digit.

Then each partner’s digits leave the rest of its own house: the line partner’s go from the rest of the line, the box partner’s from the rest of the box. A crossing digit that neither partner holds has to be in the crossing itself, so it leaves both. The pattern’s own cells keep everything.

The position is ready in our Sudoku Helper if you want to try it. Next Hint fills in the marks first; press it again for a W-Wing that removes the 3s from R4C7 and R4C8, as our solver’s earlier W-Wing had. Apply it and press again, and the Helper offers an ALS-XZ in R6C1 and row 4, the only technique it has left here. Sue de Coq is not among its eighteen, so it never names this one.

Why does Sue de Coq work?

Count. R5C9 and R6C9 need two different digits out of 2, 3, 5 and 8. They cannot take both 2 and 8, because R9C9, in the same column, would be left with nothing. They cannot take both 3 and 5, because R5C7, in the same box, would be. So the crossing takes one digit from each pair, R9C9 the other of 2 and 8, and R5C7 the other of 3 and 5: four cells, four digits, each used once. There are only four ways to fill them, and in every one the 2 and the 8 land in column 9 and the 3 and the 5 in box 6.

You never need to know which of the four is right. In the solution R5C9 is 5 and R6C9 is 8, but an 8 anywhere else in column 9, or a 5 anywhere else in box 6, is false in all four.

Seen that way, a Sue de Coq works like a naked pair or triple spread over two houses: as many cells as digits, so every digit is used exactly once. It starts from the same place as locked candidates, the cells a line shares with a box, but where locked candidates follows one digit, a Sue de Coq follows all of them at once.

What if three cells share the crossing?

Then the crossing needs five digits, and the partners still take two each, so the fifth has nowhere to go but the crossing. Puzzle 460 from the same bank has one, 24 moves in, where row 1 meets box 1: R1C1 {1,3,4}, R1C2 {1,2,8} and R1C3 {1,2,3,4,8}, with R1C7 {2,8} along the row and R2C1 {1,4} in the box. The crossing can take at most one of 2 and 8 and at most one of 1 and 4, so its third digit is the 3.

A 3 that is certainly in the crossing is in row 1 and in box 1, so it goes from both: from R1C4 in the row and from R3C3 in the box. The 2 and 8 leave R1C8, as before; the box partner’s 1 and 4 have nothing to remove.

The leftover digit does not have to be a candidate in every crossing cell: R1C2 has no 3. The count alone puts it there. All 14 three-cell Sue de Coqs in our measurement had exactly one leftover digit, and the 31 two-cell ones had none.

How do you find a Sue de Coq?

1. Go box by box. Each box is crossed by three rows and three columns, 54 crossings on the board. Skip any with fewer than two empty cells.
2. Count the crossing’s digits. You want two more than it has empty cells: four for two cells, five for three.
3. Look along the line, outside the box, for a two-candidate cell whose digits are both crossing digits.
4. Look in the box, off the line, for another made of two different crossing digits.
5. Strike the line partner’s digits from the rest of the line, the box partner’s from the rest of the box, and any leftover digit from both.

The first two steps are quick to check, but only on complete pencil marks: drop one candidate and a crossing can look as if it holds two digits too many. In our 45 moves the crossing was two cells 31 times and three cells 14 times; 25 sat on rows and 20 on columns. Most removed two to four candidates (the median was three), and only 6 left a single straight away, so expect to keep working afterwards. On the puzzles that needed one, the first came a median of 21 moves in.

How often do you need Sue de Coq?

Never on our classic levels. The generator behind Easy, Medium, Hard and Expert only deals puzzles that singles, locked candidates and naked pairs can finish, and in 200 puzzles per level a Sue de Coq was never needed.

For Evil puzzles we ran the first 500 of the bank through the helper’s eighteen techniques plus five it doesn’t have, Sue de Coq among them, always taking the simplest move. A Sue de Coq was the move 45 times, on 42 puzzles, and one was on the board at 14.4% of the positions with no single. At all 45, the helper had a move of its own: an ALS-XZ, every time. Sue de Coq comes after every helper technique except Almost Locked Sets, so that was the only one left to have. At 22 of the 45 that ALS-XZ was built from the Sue de Coq’s own cells, and at 27 it removed at least one of the same candidates.

The opening 500 Evil puzzles, a harder stretch than the bank as a whole, solved with the helper’s eighteen techniques plus these five, simplest first. Our detector finds the basic Sue de Coq described here: two or three crossing cells holding exactly two digits more than cells, and partners of up to four cells that share no digit. Measured October 2026.

So on our puzzles a Sue de Coq is an alternative, not a necessity. Learn it for the pleasure of it, and learn ALS-XZ first. On our Extreme puzzles it never comes into play at all: 499 of the first 500 stall before any of the 23 techniques makes a move.

Why is it called Sue de Coq?

After the player who posted it. In October 2005 a member of the Sudoku Players’ Forum whose handle was Sue De Coq started a thread called “Two-Sector Disjoint Subsets”: two sectors, the line and the box, and disjoint because the partners share no digit. The first reply, sixteen minutes later, read “Brilliant. A ‘Sue De Coq’.” The handle stuck as the name; the original title survives as the formal one, sometimes shortened to TSDS.

The opening post was frank about the technique’s worth: “It doesn’t crop up that often and, since we now have forced chains to crack all but the toughest puzzles, it arguably isn’t of much practical assistance. However, it’s very elegant, so here goes.” Twenty-one years on, the first 500 puzzles of our Evil bank agree on both counts.

Can’t find a Sue de Coq? Try ALS-XZ

ALS-XZ is the one to have. Our helper reached for it at all 45 positions where a Sue de Coq was our solver’s move, it was needed on 318 of the first 500 Evil puzzles, and the two overlap more than they look. At Evil 460 the helper sees no Sue de Coq but does see an ALS-XZ in the same five cells: R1C7 {2,8} as one set, the crossing plus R2C1 as the other (four cells, five digits), linked by the 2. It removes the 8 from R1C8. Linked by the 8 instead, the same two sets remove the 2. Neither reading reaches the 3, which only one set holds. That takes both links at once, and both links at once is what a Sue de Coq is.

Our Sudoku Trainer has ten ALS-XZ positions to practise on, and no Sue de Coq. Once the helper has nothing left, the work turns to chains: single-digit X-cycles, XY-chains that hop between two-candidate cells, a BUG check in the closing stages and forcing chains when all else fails. Our guide to sudoku techniques ranks them from first to last.