Advanced Sudoku begins when singles, locked candidates and subsets no longer change the grid. The next deductions usually track one candidate across several units or connect bivalue cells through explicit implications.
What are the main advanced Sudoku techniques?
The main advanced Sudoku techniques are fish patterns such as X-Wing and Swordfish, wing patterns such as XY-Wing, candidate coloring and alternating inference chains. Before using them, exhaust singles, locked candidates and subsets. Advanced techniques work by proving that a candidate must be true in one of several linked positions, allowing eliminations outside the pattern.
What should you check before an advanced technique?
Verify the givens, update every candidate, and rescan for naked singles, hidden singles, locked candidates, pairs and triples. Advanced patterns are easiest to confirm on an accurate, reduced grid. A single stale note can create a convincing false pattern.
Choose the technique from the candidate structure. Repeated positions for one digit suggest fish. Several two-candidate cells with overlapping values suggest a wing. A network of candidates appearing exactly twice in units suggests coloring or chains.
How do fish techniques work?
A fish confines one digit to matching rows and columns. In an X-Wing, two rows place candidate 6 only in the same two columns. The real 6s must occupy opposite corners, so other 6s in those columns can be removed.
Swordfish expands the structure to three rows and three columns. Jellyfish uses four. Larger fish are harder to spot and easier to misread; verify that every base-unit candidate lies inside the selected cover units.
When should you search for an X-Wing?
Follow one digit across all rows. Note rows where it appears in exactly two cells, then look for a second row sharing the same columns. Repeat with rows and columns reversed.
What is an XY-Wing?
An XY-Wing uses three bivalue cells. A pivot {X,Y} sees wings {X,Z} and {Y,Z}. If the pivot is X, the Y-wing must be Z; if the pivot is Y, the X-wing must be Z. Either way, one wing contains Z, so a cell seeing both wings cannot contain Z.
The wings do not have to see each other. Each must see the pivot, and every elimination must see both wings. Writing the candidate roles before removing anything prevents common visibility mistakes.
How is an XYZ-Wing different?
An XYZ-Wing uses a three-candidate pivot {X,Y,Z} and wings {X,Z} and {Y,Z}. The elimination must see the pivot and both wings, because Z is guaranteed somewhere among all three cells.
Are coloring and chains still logical solving?
Yes, when every implication is justified. A candidate appearing exactly twice in a unit creates a strong link: if one endpoint is false, the other must be true. Simple coloring alternates two states across connected strong links.
If two same-colored candidates see each other, that color is impossible. If an outside candidate sees both colors, it can be eliminated because one color must be true. Alternating inference chains extend the idea with strong and weak links, but the proof should remain readable from one endpoint to the other.
What should you learn first?
Learn X-Wing before Swordfish, XY-Wing before longer chains, and simple coloring before complex alternating networks. Practice one pattern until you can explain its elimination without referring to a finished solution.
Free tool / Paper-ready
Want to test the rule on your own puzzle?
Take or upload a clear Sudoku photo, review every recognized given, and choose one hint or the full solution. The editable transcription matters: correct any faint or misread digit before asking the solver to reason from the grid.
Open the Sudoku Solver from Photo →About our game: Detective Sudoku applies one-per-row-and-column deduction to suspects, rooms and truthful clues. It is not classic Sudoku, and its real hints explain a clue-driven deduction instead of giving away the answer or solution.
Frequently asked questions about advanced Sudoku
What is the easiest advanced Sudoku technique?
X-Wing is a useful first advanced pattern because it tracks one digit across two rows and two columns.
Do all hard Sudokus require advanced techniques?
No. Difficulty labels vary, and many hard puzzles resolve through accurate subsets and locked candidates.
Is a chain the same as guessing?
No. A valid chain records explicit candidate implications. Guessing chooses a branch without first proving those links.
Why can’t I find advanced patterns?
Clean candidate notes and follow one digit at a time. Many patterns remain invisible when notes are incomplete or stale.