Proven Sudoku
Techniques / Almost locked sets
ALS Chain
Also called ALS-XY-Chain, Chain of almost locked sets.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
A row of almost full groups linked one to the next. Each link passes the pressure along. The two ends then give you an elimination, the same way a chain of single squares does.
The exact rule, for stronger players
The rule, stated exactly: A sequence of almost locked sets where consecutive sets share a restricted common candidate. The endpoints share a digit z not used as a link, and z is eliminated from cells seeing every z in both end sets.
What it removes: Remove z from cells seeing all z candidates in both terminal sets.
Why it works: Each restricted common candidate forces at least one of its two sets to be locked, and the alternation propagates exactly as in an AIC with ALS nodes.
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
- ALS-XZTwo of those almost full groups share numbers.
Related almost-locked sets
- ALS-XY-WingThree almost full groups arranged like a wing.
- Death BlossomOne square is the stem.
- Subset ExclusionThe same idea for any size of group.
- Sue de CoqLook where a row or column crosses a box.
- Sue de Coq (extended)The same counting trick with more squares involved.
How often does it come up?
Our rough estimate: uncommon, and used mainly where simpler chains fail.