Puzzles · Sudoku

Sudoku: learn to see the next step

Every Sudoku you can solve by logic is a chain of small, certain steps. This page teaches five of them and lets you practise with hints that show how to think. They never fill in a digit for you.

The rules in one minute

Fill every empty cell with a digit from 1 to 9.
Each row, each column and each 3×3 box must contain every digit exactly once.
A well-made puzzle has exactly one solution, so you never need to guess.

Small pencil marks, called candidates, record which digits are still possible in a cell. Pointing Pair and Naked Pair work on these marks.

Practise

Pick a level. Easy puzzles need only the first three techniques, medium adds Pointing Pair and hard adds Naked Pair.

Keyboard: arrow keys move, 1–9 enters a digit, N switches notes, Backspace erases.

How hints work

Each press of Hint reveals a little more of the same reasoning: first where to look, then the technique's name, then the evidence, and finally a guided walk-through on your own board. You always enter the digit yourself.

Five techniques, one beat at a time

Each lesson plays in five beats: where to look, the evidence, your turn to decide, the conclusion and the rule to remember. Move at your own pace.

1
Full House
A row, column or box with one empty cell takes the only digit it is missing.
7
6
1
7
6
4
2
9
2
6
1
9
7
9
6
3
7
8
4
2
4
6
1
7
9
7
9
5
3
6
6
7
2
7
3
1
6
5
1
9
6
7
1. Look
2. Evidence
3. Your turn
4. Conclusion
5. Remember
Look at box 5: only one cell is still empty.
2
Naked Single
When the digits a cell can see rule out eight values, the ninth must go there.
7
6
1
7
6
4
2
9
2
6
1
9
7
9
6
3
7
4
2
4
6
1
7
9
7
9
5
3
6
6
7
2
7
3
1
6
5
1
9
6
7
1. Look
2. Evidence
3. Your turn
4. Conclusion
5. Remember
Look at the cell at row 4, column 6 and everything it can see: its row, column and box.
3
Hidden Single
When a digit has only one possible cell left in a row, column or box, it goes there, even if that cell has other candidates.
1
7
4
8
5
9
9
1
8
4
5
4
9
6
8
1
2
5
2
7
8
4
3
9
9
1
3
6
4
4
7
6
5
8
5
2
9
4
3
1
6
7
1. Look
2. Evidence
3. Your turn
4. Conclusion
5. Remember
Look at row 1: where can 6 go?
4
Pointing Pair
If a digit's only places in a box lie on one line, that digit can be removed from the rest of that line.
7
3
4
1
8
9
5
5
3
4
7
1
5
4
8
3
9
2
3
5
3
3
7
9
5
6
3
2
1
8
9
4
7
4
6
3
5
1
7
5
9
3
6
8
1. Look
2. Evidence
3. Your turn
4. Conclusion
5. Remember
Look at box 9: where can 2 go?
5
Naked Pair
Two cells of a unit with the same two candidates claim those digits, so the other cells of the unit lose them.
9
3
1
5
2
7
6
5
1
8
7
2
8
5
8
2
4
9
1
5
8
3
9
1
6
2
8
9
5
9
5
6
1
5
7
6
9
1
1. Look
2. Evidence
3. Your turn
4. Conclusion
5. Remember
Look at box 1.