Proven Sudoku
Techniques / Almost locked sets
Almost Hidden Set
Also called AHS.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
The mirror image of an almost locked set. Instead of squares holding one spare number, it is numbers holding one spare square. It links into chains the same way.
The exact rule, for stronger players
The rule, stated exactly: A set of n digits confined to n+1 cells within a single house. Removing one cell from consideration leaves a hidden set.
What it removes: Nothing alone. Used as a chain node, exactly as an ALS is.
Why it works: The dual of the ALS. n digits needing n cells have one spare, so eliminating one cell's participation locks the rest.
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
- Hidden Subset (general form)Some numbers in a group have only a few homes left.
Related almost-locked sets
- Almost Locked CandidatesTwo areas cross.
- Almost Locked SetA small group of squares holding exactly one number more than there are squares.
- ALS ChainA row of almost full groups linked one to the next.
- ALS-XY-WingThree almost full groups arranged like a wing.
- ALS-XZTwo of those almost full groups share numbers.
How often does it come up?
Our rough estimate: less used than the ALS, mostly because ALS is easier to see on a pencil-marked grid.