Sudoku Techniques

Updated 23 August 2026

A Sudoku technique is a named pattern of candidates that justifies placing a digit or erasing a candidate without guessing. This page lists the techniques in the order a solver should try them, from the single cheapest deduction in the game to the subset patterns that unlock most mid-difficulty puzzles.

Every technique page here carries a worked example board that was produced by running the solving engine bundled with the Suuudokuuu app over a real puzzle position. The highlighted pattern cells, the eliminated candidates and the placement under each board are the engine’s own output, so the explanation and the diagram can never drift apart.

Tier 1 — Singles

Singles place a digit directly. They need no elimination pattern, they never remove a candidate without also solving a cell, and a puzzle solvable with singles alone is what most apps call easy.

• Full House — the last empty cell in a row, column or box takes the one digit that unit is still missing.
• Naked Single — a cell whose row, column and box between them rule out eight digits, leaving one candidate.
• Hidden Single — a digit that fits in only one cell of a unit, even though that cell still has other candidates.

Tier 2 — Intersections

Intersection techniques work on the overlap between a box and a line. They never place a digit on their own; they remove candidates, and those removals usually expose a single on the next pass.

• Pointing Pair — a digit confined to two cells of a box that share a line is erased from the rest of that line.
• Pointing Triple — the same deduction with three cells instead of two.
• Box Line Reduction — the mirror image: a digit confined to one box along a line is erased from the rest of that box.

Tier 3 — Subsets

Subsets reserve a group of digits for a group of cells. Naked subsets are found by looking at cells and counting the digits they can hold; hidden subsets are found by looking at digits and counting the cells that can hold them. Both are elimination techniques, and both scale from two cells up to four.

• Naked Pair — two cells in a unit hold the same two candidates and nothing else.
• Naked Triple — three cells whose candidates together use only three digits.
• Naked Quad — four cells whose candidates together use only four digits.
• Hidden Pair — two digits that fit in only two cells of a unit.
• Hidden Triple — three digits confined to the same three cells.
• Hidden Quad — four digits confined to the same four cells.

Tier 4 — Fish, wings and chains

Beyond the subsets, techniques stop reasoning about a single unit and start reasoning across two or more units, or across a path of cells, at once. Fish patterns trap a digit in the same set of rows and columns; wings link a small handful of cells through shared candidates; chains follow strong and weak links as far as the board allows. These are the techniques that carry the hardest puzzles Suuudokuuu generates.

• X-Wing — a digit confined to the same two columns across two rows is erased from the rest of those columns.
• Swordfish — the same fish pattern spread across three lines instead of two.
• Jellyfish — the same fish pattern spread across four lines.
• Finned X-Wing — an X-Wing with one extra candidate that still supports a smaller, safe set of eliminations.
• Finned Swordfish — the same fin logic applied to a Swordfish.
• Sashimi X-Wing — a finned X-Wing where one base line is missing its normal corner candidate entirely.
• Sashimi Swordfish — the same missing-corner pattern applied to a Swordfish.
• XY-Wing — three bivalue cells arranged so a shared candidate can be erased from any cell that sees both pincers.
• XYZ-Wing — an XY-Wing whose pivot also carries the shared candidate, tightening the elimination further.
• W-Wing — two matching bivalue cells tied together by a strong link on one of their shared candidates.
• X-Chain — a chain of strong and weak links on one digit connecting two cells that share a peer.
• XY-Chain — a chain of bivalue cells whose two endpoints force an elimination wherever they overlap.
• Simple Coloring — a two-color network of strong links on one digit, cleared wherever a cell sees both colors.
• AIC — an alternating inference chain that generalises the fish, wing and chain patterns above into one technique.

How to use this list

Work down the list, not across it. A solver that reaches for a naked quad while an unspotted hidden single is sitting on the board is doing far more work than the puzzle asks for, and the same is true of a human. Suuudokuuu applies the techniques in exactly the order above and stops at the first one that fires, which is also how the app decides what to show when you ask for a hint.

The fish patterns, the wings and the chains above follow the same principle as the subsets — reserving digits for cells — but they reason across two or more units at once, or across a path of cells, rather than inside a single unit.