Use 1–9 once
Across the board, a horizontal row cannot repeat a digit. If a row already contains 7, another 7 cannot go anywhere in that row.
Classic 9×9 rules
Sudoku rules: rows, columns,
Sudoku rules: rows, columns,
and 3×3 boxes.
The three Sudoku rules are simple: place digits one through nine so every row, every column, and every 3×3 box contains each digit exactly once. The puzzle is logic, not arithmetic.
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The three Sudoku rules
Every legal placement passes all three checks.
If a value creates a duplicate in any one of these views, it does not belong in that square.
Use 1–9 once
Down the board, a vertical column cannot repeat a digit. A candidate must survive the full height of its column.
Use 1–9 once
Inside each small 3×3 region, the same one-of-each rule applies. A value needs all three permissions—row, column, and box.
The important distinction
Sudoku is not arithmetic.
The digits behave like symbols. Their numerical value does not matter; the logic comes from whether a symbol is already present in the surrounding row, column, or box. That is why the same mental model can later expand into Beyond’s larger 16×16 field.
- ALook at a unit
Start with the row, column, or box that has the most given numbers.
- BList the missing digits
Turn a crowded unit into a constrained shortlist.
- CCheck the crossings
Use the other two views to eliminate values that cannot fit.
Sudoku rules FAQ
Can a number repeat in a row, column, or box?
No. In classic 9×9 Sudoku, each digit from 1 to 9 appears once in every row, once in every column, and once in every 3×3 box.
Do the digits have to add to a target?
No. The digits work as symbols. Their arithmetic value is irrelevant; only duplicates and legal placement matter.
What should a new player do first?
Find a crowded row, column, or box, list the missing digits, and use the crossing constraints to remove impossible values.