Three rules
Row
Every row must contain 1–9, with each digit appearing once.
Column
Every column must contain 1–9, with each digit appearing once.
Box
Every 3 × 3 box outlined by thicker lines must contain 1–9, with each digit appearing once.
How to check a move
Choose an empty cell and a digit, then check:
The digit is not already in the row.
The digit is not already in the column.
The digit is not already in the 3 × 3 box.
Candidates and notes
Candidates are digits missing from a cell’s row, column and box. Write them as notes. After placing a digit, remove it from related cells’ notes. If one candidate remains, fill it in. Notes are not answers.
Move examples
Valid move
No 5 appears in this row, column or box, so entering 5 creates no duplicate. Further reasoning is still needed to confirm the answer.
Conflict
Row 1 already contains a 5, and another 5 is entered in that row. The repeated digit must be removed. The same rule applies to columns and boxes.
Common questions
Does Sudoku require arithmetic?
No. Digits are symbols. Solving depends only on what is repeated or missing in each row, column, and box.
Can digits repeat on a diagonal?
Yes, in standard Sudoku. Diagonals matter only when a puzzle explicitly adds a diagonal rule.
Can a Sudoku have more than one solution?
A proper standard Sudoku should have exactly one solution. Multiple solutions mean the clues are insufficient.