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Sudoku Rules

Learn the rules of classic 9×9 Sudoku.

Classic Sudoku has three core constraints: every completed row, every completed column, and every completed 3×3 box must contain the digits 1–9 exactly once.

That is the rule set. Scanning, candidates, pairs, X-Wings, Chains, and other solving techniques do not add new rules; they are different ways of extracting consequences from the same constraints.

Sudoku visualThe three units that constrain every cellEvery placement must respect its row, column, and 3×3 box at the same time.
Row
5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Column
5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Box
5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
  • Row
  • Column
  • Box

Rule 1: Every row contains 1–9

Each horizontal row has nine cells. When the Sudoku is complete, that row must contain each digit from 1 through 9 exactly once.

If a row already contains a 6, no other cell in that row can be 6.

That immediately lets you rule out positions whenever you inspect an empty cell.

Rule 2: Every column contains 1–9

The same rule applies vertically.

Each column must contain the digits 1 through 9 exactly once in the completed puzzle. If column 4 already contains an 8, every other unsolved cell in column 4 can exclude 8.

Rule 3: Every 3×3 box contains 1–9

A classic Sudoku grid is divided into nine 3×3 boxes. Each box also contains the digits 1 through 9 exactly once when solved.

This means an empty cell is controlled by three overlapping units at the same time: its row, its column, and its box.

That overlap is what creates most Sudoku deductions.

You cannot change the givens

A Sudoku begins with some digits already supplied. VeyraPlay calls these givens. They are part of the puzzle and stay fixed.

Your entries fill the remaining cells.

In the game UI, givens and player-entered digits should always be visually distinguishable, but the underlying Sudoku rules treat both as solved digits once their value is established.

What counts as a valid placement?

Before placing a digit in an empty cell, ask three questions:

  1. Is that digit already in the row?
  2. Is that digit already in the column?
  3. Is that digit already in the box?

If the answer to any of them is yes, the placement is not allowed.

If the answer to all three is no, the digit is possible in that cell — but that does not automatically prove that it belongs there. It may simply be one candidate among several.

That distinction is important:

  • legal candidate: the digit is not ruled out yet
  • solved digit: the logic proves that the digit must be placed there

Do diagonals matter in normal Sudoku?

No. In classic Sudoku, the two long diagonals do not have to contain 1–9 uniquely.

Some variants add diagonal rules, but those are separate rule sets. Unless a puzzle explicitly says otherwise, only rows, columns, and boxes matter.

Do the numbers have to add up to anything?

No. Standard Sudoku has no sum rule.

The digits are labels from 1 to 9. Their numerical value does not create an arithmetic condition.

Can the same digit appear more than once in the whole grid?

Yes. Every digit appears multiple times across the completed grid.

The restriction is local: a digit cannot repeat within the same row, column, or box. A completed classic Sudoku contains each digit nine times overall — once in every row, once in every column, and once in every box.

A quick rules example

Are pencil marks part of the rules?

No. Pencil marks are a solving aid.

They record candidates inside unsolved cells, which can make harder deductions easier to see. You can solve many easy Sudokus without writing candidates at all.

Are solving techniques extra rules?

No. Techniques are logical consequences of the same three basic restrictions.

A Naked Single, for example, happens when eight digits have been ruled out of a cell, leaving only one candidate. An X-Wing is more advanced, but its eliminations still come from the fact that each digit must occupy a valid position in the relevant rows and columns.

FAQ

Can zero be used in Sudoku?

Not in standard 9×9 Sudoku. Classic Sudoku uses the digits 1 through 9.

Can a row contain two of the same digit while the puzzle is unfinished?

A correct puzzle state cannot. If two identical placed digits appear in the same row, at least one player entry is wrong.

Does every 3×3 box need every digit from 1 to 9?

Yes. Each box contains 1–9 exactly once in a completed classic Sudoku.

Is guessing against the rules?

Guessing is not one of the structural rules, but it is different from logical solving. VeyraPlay Guides and Learn are designed around deductions you can justify from the grid.

What to learn next

Once the rules make sense, learn the Sudoku Grid terminology and then move to How to Play Sudoku, where you will use these rules to make your first deductions.

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