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Quick answer
Sudoku is a free number-placement logic puzzle that plays instantly in your browser on phones, tablets and computers, with no download, sign-up or ads. Fill the 9×9 grid so every row, column and 3×3 box holds the digits 1 to 9 once.
Sudoku at a glance
What it is
Number-placement logic puzzle
This page
Hard level: Locked candidates, pairs or triples
Goal
Fill the 9×9 grid so every row, column and 3×3 box holds the digits 1 to 9 once
Modes
Easy, Medium, Hard and Expert, graded by the hardest solving technique needed; every puzzle has exactly one solution
Controls
Tap a cell and a number, or type 1-9; notes mode, auto notes, mistake highlighting, hints, undo
Players
1
Price
Free, no ads, no sign-up, no download
Works on
Phones, tablets and computers, in any modern browser
In a hard puzzle you will reach a point where no digit can be placed directly. The answer is to remove candidates until a single appears. That is why notes matter: press Fill notes once and keep the pencil marks tidy as you go.
The techniques you will need
Locked candidates (pointing). In a box, if every note for a 4 lies in the same row, the 4 for that box must be in that row. No other cell of the row outside the box can be a 4.
Locked candidates (claiming). In a row, if every note for a 4 lies inside one box, the rest of that box cannot hold a 4.
Naked pairs and triples. Two cells in a unit with the notes {3, 8} and {3, 8} lock up both digits. Remove 3 and 8 from every other cell in that unit. Three cells whose notes use only three digits work the same way.
Hidden pairs and triples. If the digits 2 and 6 can only go in the same two cells of a unit, those cells hold 2 and 6, and every other note in those two cells can be removed.
Strategy tips
Always clear singles first. They are the cheapest progress, and each one may expose a pair.
Scan boxes for pointing pairs. They are the most common hard-level step and are quick to spot once notes are filled in.
Work in one unit at a time. Count how many cells each digit can go in. A digit with two places in two units often points somewhere useful.
Mastered these? Expert Sudoku needs X-Wings and XY-Wings.
How to play Sudoku
1
Fill in candidates
Press Fill notes to pencil every possible digit into every empty cell. Hard puzzles are much easier to read with notes.
2
Place the singles
Place every naked and hidden single you can find. Notes in the same row, column and box clear automatically.
3
Look for locked candidates
If a digit's notes in a box all sit in one row or column, remove that digit from the rest of that row or column outside the box.
4
Look for pairs
Two cells in a unit with the same two candidates hold those two digits between them, so remove both digits from the other cells of the unit.
The hardest level. Each puzzle needs an X-Wing, XY-Wing or Swordfish, and still never a guess.
Frequently asked questions
What makes a Sudoku hard here?
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Every hard puzzle needs at least one technique beyond singles, such as locked candidates or a naked or hidden pair or triple, and none of the expert techniques. Most have between 24 and 29 clues.
Do hard puzzles need guessing?
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No. Each one is solved by a logic engine before you see it, using only the techniques listed here. That also proves it has exactly one solution.
What are locked candidates?
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When all the places for a digit inside a box lie in a single row or column, that digit must be in that box's part of the line. You can remove it from the rest of the line. It also works the other way, from a line into a box.
What is a naked pair?
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Two cells in the same row, column or box that both have exactly the same two candidates. Those two digits must go in those two cells, so they can be removed from every other cell in that unit.
Does a hint tell me which technique to use?
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Yes. If the next cell can only be found after an elimination, the hint shows the cell and its digit and names the technique that leads there, for example a naked pair.
Fancy a different puzzle?
Minesweeper is logic of another kind: read the numbers and clear the field without opening a mine.