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Sudoku Tips & Strategy

A practical Sudoku strategy guide: scanning and cross-hatching, naked and hidden singles, naked pairs, pointing pairs, box/line reduction, and X-wing — the techniques that solve Easy through Expert.

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The core idea

Every step is forced — find the one that is

Sudoku is not about trying digits until one fits. In any solvable position there is a cell that can hold only one digit, or a digit that can go in only one cell of some unit. The techniques below are ordered by how hard that forced move is to see — learn them in order.

  • 01

    Scanning and cross-hatching

    Pick a digit and look at where it already appears. Each placed copy rules out its whole row and column. Cross-hatch a 3×3 box by projecting those lines into it — often only one cell in the box is left open for that digit. This alone solves most Easy puzzles and opens Medium ones.

  • 02

    Naked singles

    For an empty cell, cross out every digit that appears in its row, column, or box. If exactly one digit survives, it must go there. Turning on notes makes naked singles visible at a glance — a cell showing a single pencil mark is done.

  • 03

    Hidden singles

    Scan one unit — a row, column, or box — for a digit that can legally land in only one of its cells, even if that cell still has several candidates. The digit is "hidden" among other notes but is just as forced as a naked single.

  • 04

    Naked pairs and triples

    If two cells in a unit share the exact same two candidates, those two digits are locked to that pair. Erase them from every other cell in that unit. The same logic extends to three cells sharing three candidates.

  • 05

    Pointing pairs (box/line reduction)

    If, inside one box, a digit’s only candidates all sit in a single row or column, that digit must come from the box on that line — so remove it as a candidate from the rest of that row or column outside the box. The reverse also works: a digit confined to one box within a line clears the rest of the box.

  • 06

    X-wing

    When a digit’s candidates in two rows are limited to the same two columns, the digit forms an X across those four cells. It must occupy one cell per row within those columns, so it can be eliminated from the rest of both columns. Swap rows and columns for the mirror version. This is the tool that breaks a stuck Expert grid.

Habits

Four habits that prevent mistakes

The techniques find moves; these habits keep you from throwing a good grid away. They matter more as difficulty climbs. See the how-to-play guide for the full rules.

  • Notes on above Easy

    From Medium upward, pencil in candidates before you try to place anything. Holding nine constraints in your head is where mistakes come from.

  • Prune after every placement

    The moment you place a digit, remove it from the notes of its row, column, and box. Stale candidates are the number-one reason a naked single gets missed.

  • Work the fullest units first

    Rows, columns, and boxes that already hold six or seven digits are where the next forced move usually hides. Start there rather than in the emptiest corner.

  • Never guess

    A guess can send you moves deep into an unsolvable branch. If you cannot find a logical step, re-scan the digits you have placed most often — the pattern is there.

Ready to put it into practice? Read the full how-to-play guide or jump straight in.

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