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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
Medium level: Hidden singles
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
Medium puzzles give you fewer clues, about 30, and they are built so that looking cell by cell is no longer enough. Somewhere in every medium puzzle you will reach a point where no cell has a single option left. That is where hidden singles come in.
Hidden singles, step by step
Instead of asking “which digit fits this cell?”, ask “where can this digit go in this box?”
Choose a box and a digit it is still missing, say 5.
Look along the rows and columns that pass through the box. Every 5 already placed there rules out a whole line of cells.
If only one empty cell in the box is left for the 5, that is where it goes.
The same works for a row or a column. People usually find hidden singles faster than naked singles once they get used to them, because you can sweep one digit across the whole grid.
Strategy tips
Sweep one digit at a time. Pick a digit and check all nine boxes for it before moving on.
Start with the digits you see most. A digit with seven or eight copies placed has very few spots left.
Use Fill notes for a full picture. It pencils every candidate into every empty cell, and notes clear automatically as you place digits.
Ready for more? Hard Sudoku introduces pairs and locked candidates.
How to play Sudoku
1
Know the goal
Fill every empty cell so that each row, each column and each 3×3 box contains the digits 1 to 9 exactly once.
2
Find naked singles
Look for cells where every digit but one is already used in the cell's row, column or box.
3
Find hidden singles
Pick a box, row or column and a digit it is missing. If that digit fits in only one of its empty cells, it goes there.
4
Keep notes
Turn on Notes to pencil in candidates, or press Fill notes to add every possible digit at once, then clear them as you go.
The hardest level. Each puzzle needs an X-Wing, XY-Wing or Swordfish, and still never a guess.
Frequently asked questions
How is medium different from easy?
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Medium puzzles start with about 30 clues instead of 38, and they cannot be finished with naked singles alone. At some point you will need a hidden single.
What is a hidden single?
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A digit that has only one possible place in a row, column or box. The cell may still have other candidates, but the digit has nowhere else to go, so it must go there.
Will I ever have to guess?
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No. Every medium puzzle is generated and checked so that it can be solved with naked and hidden singles alone, and it has exactly one solution.
What does the Hint button tell me?
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Hint points to a cell you can fill next and names the technique, for example the only place for a 7 in box 4. Press Fill to place it, or work it out yourself.
Can I play with a keyboard?
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Yes. Use the arrow keys to move, 1 to 9 to enter a digit, Shift and a digit for a note, N to switch notes mode, 0 or Delete to erase, H for a hint and Ctrl+Z to undo.
Fancy a different puzzle?
Minesweeper is logic of another kind: read the numbers and clear the field without opening a mine.