Sudoku techniques

First needed on

Easy Sudoku

• Last empty cell

  The last empty cell of a row, column or box takes the one digit missing.

• Hidden single

  A digit with only one place left in a row, column or box.

  Guide with a worked example: Hidden single

• Naked single

  A cell whose row, column and box rule out all but one digit.

  Guide with a worked example: Naked single

First needed on

Hard Sudoku

• Pointing

  A digit locked to one line inside a box leaves the rest of that line.

  Guide with a worked example: Pointing

• Box-line reduction

  A digit locked to one box on a line leaves the rest of that box.

  Guide with a worked example: Box-line reduction

• Naked pair

  Two cells of a unit with the same two candidates take both digits.

  Guide with a worked example: Naked pair

• Hidden pair

  Two digits that fit only in the same two cells of a unit.

  Guide with a worked example: Hidden pair

First needed on

Expert Sudoku

• Naked triple

  Three cells of a unit that between them hold only three digits.

  Guide with a worked example: Naked triple

• Hidden triple

  Three digits that fit only in the same three cells of a unit.

• Naked quad

  Four cells of a unit that between them hold only four digits.

• Hidden quad

  Four digits that fit only in the same four cells of a unit.

• X-Wing

  A digit on two rows confined to the same two columns, or the other way round.

  Guide with a worked example: X-Wing

First needed on

Master Sudoku

• XY-Wing

  A two-candidate pivot and two wings that force a digit out where both wings see.

  Guide with a worked example: XY-Wing

• XYZ-Wing

  An XY-Wing whose pivot also holds the shared digit.

• Swordfish

  The X-Wing with three rows and three columns.

  Guide with a worked example: Swordfish

• Jellyfish

  The X-Wing with four rows and four columns.

Questions

Which techniques do I need for each level?

Easy and medium puzzles here need only singles. Hard adds pointing, box-line reduction and pairs. Expert adds triples, quads and the X-Wing. Master needs at least one XY-Wing, XYZ-Wing, Swordfish or Jellyfish.

Do I ever need to guess?

Not on this site. Every puzzle we list was solved to the end by our solver using only the techniques on this page, so a logical path always exists.

Where do the examples come from?

From our own puzzles. Each guide builds one of the puzzles you can play here, runs our solver on it step by step and stops at the first step that needs the technique, then shows the board at that moment.