Sudoku Rules

Sudoku is a logic puzzle built around a simple set of constraints.

A standard Sudoku uses a 9×9 grid divided into nine 3×3 boxes. Some cells already contain numbers, while the remaining cells must be completed by the player.

To solve the puzzle correctly, every final placement must follow three core rules:

1. Each row must contain the numbers 1 through 9 exactly once.
2. Each column must contain the numbers 1 through 9 exactly once.
3. Each 3×3 box must contain the numbers 1 through 9 exactly once.

Those three rules define standard Sudoku.

You do not need arithmetic, and you should not need random guessing. The puzzle is solved by finding placements that satisfy all three constraints at the same time.

The Sudoku grid

A standard Sudoku contains 81 cells arranged in nine rows and nine columns.

The grid is also divided into nine smaller regions called boxes, blocks, or subgrids. Each box contains 3×3 cells.

This means that every cell belongs to:

• one row;
• one column;
• one 3×3 box.

That overlap is what creates the puzzle.

A number may look possible when you only consider its row, but become impossible when you check its column or box.

Every placement must therefore satisfy all three groups simultaneously.

Rule 1: Each row must contain 1 through 9

Every horizontal row must contain each number from 1 to 9 exactly once.

A row cannot contain duplicate numbers.

The order does not matter. This is also valid:

What matters is that every number from 1 through 9 appears once and only once.

When considering a possible number for an empty cell, always check the rest of its row.

Rule 2: Each column must contain 1 through 9

The same rule applies vertically.

Every column must contain each number from 1 through 9 exactly once.

If a column already contains a 6, no other cell in that column may contain another 6.

4
7
1
9
3
6
2
5
8

This is a valid completed column.

A number that is allowed by a cell's row can still be invalid if the same number already appears somewhere in its column.

This is why Sudoku cannot be solved by looking at rows alone.

Rule 3: Each 3×3 box must contain 1 through 9

Every one of the nine 3×3 boxes must also contain each number from 1 to 9 exactly once.

1  5  9
4  8  2
7  3  6

This is a valid completed box because all nine digits appear exactly once.

Just like rows and columns, boxes cannot contain duplicates.

A valid Sudoku move must satisfy all three rules

A Sudoku move is only valid if the number does not duplicate another copy of the same number in its:

Suppose an empty cell could contain a 4 based on its row.

If the column already contains a 4, you cannot place it there.

If the column is clear but the 3×3 box already contains a 4, you still cannot place it there.

A candidate is only genuinely possible when it survives all three checks.

This three-way constraint is the foundation of every Sudoku solving technique.

What are clues or givens?

The numbers already present when a Sudoku begins are called clues or givens.

These cells are fixed.

They form the starting state of the puzzle and must not be changed during normal play.

The clues determine the structure of the puzzle and provide the information needed to deduce every remaining number.

A valid Sudoku puzzle should contain enough information to lead logically to its intended solution.

The number of starting clues can vary significantly between puzzles. However, clue count alone does not determine difficulty. Two puzzles with the same number of givens can require very different solving techniques.

Can the same number appear more than once in Sudoku?

Yes — across the entire grid.

A completed standard Sudoku contains each number from 1 through 9 nine times in total.

The restriction only applies within individual units.

A number cannot appear twice in the same:

• row;
• column;
• 3×3 box.

For example, two different rows may each contain a 5. That is completely normal.

They simply cannot place those 5s in positions that create a duplicate within the same column or box.

Can you change a clue in Sudoku?

No.

The original clues are fixed parts of the puzzle.

Changing one changes the puzzle itself and may make the intended solution impossible or create a different puzzle entirely.

When playing digitally, clue cells are normally locked so that they cannot be edited accidentally.

Numbers you enter yourself are different. Those can usually be changed or removed while solving.

Can you use notes in Sudoku?

Yes.

Notes, often called pencil marks, are not final placements.

They represent possible candidates for an empty cell.

That means all three values are currently possible based on the information available.

Using notes does not break Sudoku's rules because candidates are not treated as placed numbers.

As the puzzle develops, candidates can be eliminated until only one valid value remains.

Do you have to fill the numbers in order?

No. There is no required solving order in Sudoku.

You do not need to complete:

• the first row before the second;
• one box before moving to another;
• the numbers in numerical order.

You are free to work anywhere on the grid.

In fact, moving between different rows, columns, and boxes is usually more effective than trying to complete one area at a time.

Is guessing allowed in Sudoku?

Nothing physically prevents you from guessing, but a properly constructed Sudoku is intended to be solved through logical deduction.

If two numbers seem equally possible in a cell, the best approach is usually to leave it unresolved and look for more information elsewhere.

Guessing can introduce errors that only become visible much later.

Veyra Sudoku is intended to favour puzzles that can be solved logically without relying on random trial and error.

Advanced solving may involve complex chains of reasoning, but those deductions are still based on the constraints of the puzzle.

Does every Sudoku have one solution?

A well-constructed standard Sudoku puzzle should have a unique solution.

That means there is exactly one completed 9×9 grid that satisfies all the clues and Sudoku rules.

If a puzzle allows multiple valid solutions, it is generally considered ambiguous rather than a properly formed standard Sudoku puzzle.

The existence of a unique solution is important because it means every correct placement can ultimately be justified by the information contained in the puzzle.

Can a Sudoku puzzle be impossible?

Yes.

A puzzle can become impossible if:

• its original clues contradict each other;
• clues make a valid completion impossible;
• the player enters an incorrect number and treats it as correct;
• the puzzle was generated incorrectly.

For example, if the starting clues already contain two 7s in the same row, the puzzle violates Sudoku's rules before play even begins.

A properly generated puzzle should begin in a valid state and have at least one solution — ideally exactly one.

Veyra Sudoku's puzzle pipeline is intended to validate generated puzzles before they are presented to players.

When is a Sudoku complete?

A Sudoku is complete when every cell has been filled and all three rules remain satisfied.

• Every row contains 1 through 9 exactly once.
• Every column contains 1 through 9 exactly once.
• Every 3×3 box contains 1 through 9 exactly once.

A completely filled grid is not automatically a correct solution.

If even one row, column, or box contains a duplicate, the puzzle is not solved correctly.

What happens if you make a mistake?

In a traditional paper Sudoku, an incorrect number simply becomes part of your working grid until you notice and correct it.

Digital Sudoku games can handle mistakes in different ways.

Depending on the game mode, Veyra Sudoku may eventually support options such as:

• highlighting conflicting numbers;
• limited mistake counters;
• validation only when the puzzle is complete;
• unrestricted play without immediate error feedback.

Those are interface and gameplay settings rather than changes to the underlying Sudoku rules.

Are diagonal numbers part of the standard rules?

No.

In standard Sudoku, the two main diagonals do not need to contain the numbers 1 through 9 exactly once.

Only rows, columns, and 3×3 boxes matter.

However, some Sudoku variants introduce additional rules. For example, Diagonal Sudoku may require one or both long diagonals to contain 1 through 9 without repetition.

Those extra constraints belong to the specific variant and are not part of standard Sudoku.

Are there different Sudoku rules for different difficulties?

No.

Easy, Medium, Hard, and Expert Sudoku all follow the same fundamental rules.

Difficulty changes because of the logic required to discover the solution, not because the underlying rules change.

An Easy Sudoku may offer many obvious placements. A Hard or Expert puzzle may require you to track candidates and use more advanced solving techniques.

You can learn more about these methods in our Sudoku Strategies guide.

Rules vs. solving strategies

The rules tell you what a valid Sudoku must look like.

Strategies tell you how to discover which number belongs where.

Once you're comfortable with the rules, continue with our How to Play Sudoku guide for the basic solving process or explore Sudoku Strategies for specific logical techniques.

Frequently asked questions