Lesson 1 · Start here
Sudoku Rules
Learn the three rules that make every valid Sudoku move possible.
See the logic move
Use Next to reveal each reason. On the final step, tap the highlighted cell to complete the move yourself.
Step 1 of 3
Check across
The target cell is in row 5. Scan every number already in that row.
Select an empty cell to see its legal candidates and why the other digits are blocked.
- 1
Focus: Start here. This is the cell or area being investigated. R5C5.
- 2
Relevant cells: These cells share the row, column, or box used by this step. Row 5.
What the board is showing
Explain it simply
Your job is to place 1 to 9 so no number repeats in a row, column, or 3x3 box.
Step by step
- Check the row.
- Check the column.
- Check the 3x3 box.
- Place the number only when all three allow it.
From demonstration to independent solving
Prove the move yourself
What you should be able to do
Finish this lesson able to test any proposed number against all three constraints that control its cell.
Read the exact board evidence
At R5C5, row 5 already contains 4, 8, 3 and 1; column 5 contains 7, 9, 6, 2, 1 and 8; the centre box contains 6, 8, 3 and 2. Combining those three inventories leaves 5 as the only digit not blocked.
When the logic is strong enough
A placement is valid only when the chosen digit is absent from the cell's row, column and box. Passing one or two checks is not enough.
Train the pattern without guessing
Choose three empty cells in an Easy puzzle. Without placing anything, say the row, column and box numbers aloud and list which digits each unit blocks.
Quick check: Can a number appear twice in one 3x3 box if it is in different rows?
No. Every 3x3 box must contain each number only once.
Turn logic into skill