Master Sudoku Techniques
One floor below expert: almost locked sets, the long chains and loops that argue from strong and weak links, the fish that break the row-and-column rule, and the one honest way to guess. All nine are here, in the order they are best learned.
- Almost Locked SetsCells in one unit holding exactly one shape more than they have room for. Hand the set any one of those shapes from outside and the rest lock.
- ALS-XZTwo almost locked sets sharing two shapes. One of the two shapes can land in only one of the sets, which locks the other — so the second shape is trapped inside the pair.
- ALS ChainsThree almost locked sets in a row, each handing the next a shape it cannot keep. Whichever end locks, one shape is used up inside the chain.
- Sue de CoqA box/line intersection and one helper set in each unit, together using up their whole pool of shapes — so the pool clears out of both units at once.
- Nice LoopsA chain closed into a loop. Where the alternation survives the join, every weak link in it starts eliminating.
- Forcing ChainsAssume each candidate of one cell in turn, follow what each one forces, and strike whatever every branch rules out — no alternation needed, only agreement.
- Franken and Mutant JellyfishA four-set fish that lets a 3×3 box stand in for a line, on either side of the count.
- Kraken FishA finned fish whose fin is carried on by a chain, so the strike lands on a cell the fin cannot see. The hardest argument on the site.
- Trial and ErrorThe one move that is not a deduction: assume a candidate, follow what it forces, and strike it when the grid contradicts itself.
Coming from below? The expert techniques are where the fish, the fins and the uniqueness arguments live, and every technique on this tier is built on one of them. If you would rather have the work done for you, the sudoku solver walks any grid one technique at a time and names each move.