Proven Sudoku
Techniques / Almost locked sets
Sue de Coq
Also called SDC, Two-Sector Disjoint Subsets, Sue-de-Coq.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Look where a row or column crosses a box. Count the numbers those shared squares can hold against the room available. When the count is exactly full, everything else in both the row and the box gives way.
The exact rule, for stronger players
The rule, stated exactly: Cells in the intersection of a box and a line carry candidates that split into two disjoint groups, one completed by an ALS in the box and the other by an ALS in the line. Digits used by the intersection are then eliminated from the rest of both houses.
What it removes: Remove the involved digits from the box outside the intersection and from the line outside the intersection, according to which set claims them.
Why it works: The intersection cells consume a known set of digits between them. Whatever they do not take is forced into the two flanking sets, which saturates both houses for those digits.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Almost Locked SetA small group of squares holding exactly one number more than there are squares.
- Locked Candidates (general form)Sometimes all the spots for a digit inside one group also sit inside a second group.
Related almost-locked sets
- Sue de Coq (extended)The same counting trick with more squares involved.
- Aligned Pair ExclusionLook at two squares together and list every pair of digits they could hold.
- Aligned Triple ExclusionLook at three squares together and list every set of three numbers they could hold.
- Almost Hidden SetThe mirror image of an almost locked set.
- Almost Locked CandidatesTwo areas cross.
How often does it come up?
Our rough estimate: uncommon but powerful, often producing many eliminations at once.