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Classic Sudoku has a simple rule set: fill the grid with digits 1–9 so that every row, every column, and every 3×3 box contains each digit exactly once.
Those three overlapping constraints are the foundation of the entire puzzle. Scanning, candidates, pairs, Fish, Wings, Coloring, and Chains are solving techniques, not additional rules. They are different ways of proving consequences from the same constraints.
- Row
- Column
- Box
The rules of Sudoku at a glance
A classic 9×9 Sudoku has 81 cells. Some cells begin with fixed digits called givens or clues; you fill the remaining cells.
A completed grid must satisfy all three rules simultaneously:
- Rows: every row contains 1–9 exactly once.
- Columns: every column contains 1–9 exactly once.
- 3×3 boxes: every box contains 1–9 exactly once.
A digit that is allowed by the row but blocked by the column or box is still not legal in that cell. Every placement must respect all three units.
Rule 1: Every row contains 1–9
Each horizontal row has nine cells. When the Sudoku is complete, the 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 contain 6. That lets you remove 6 as a possibility from every other unsolved cell in the same row.
Conversely, if a row is missing only one digit, that final digit has only one place left in the row. This simplest kind of forced completion is often described as a last missing digit or Full House.
Rule 2: Every column contains 1–9
The same restriction applies vertically.
Every completed column must contain the digits 1 through 9 exactly once. If column 4 already contains an 8, every other unsolved cell in column 4 can exclude 8.
Rows and columns constantly interact. A digit that appears to fit an empty position from the row alone may be ruled out as soon as you inspect its column.
Rule 3: Every 3×3 box contains 1–9
The classic Sudoku grid is divided into nine 3×3 boxes. Every completed box must also contain each digit from 1 through 9 exactly once.
That means an empty cell is constrained by three overlapping units at the same time: one row, one column, and one box.
The box rule is what beginners most often forget when they first learn Sudoku. A digit can be absent from both the row and column and still be impossible because the same digit already appears elsewhere in the 3×3 box.
The overlap among rows, columns, and boxes creates most Sudoku deductions.
You cannot change the givens
A Sudoku begins with some digits already supplied. VeyraPlay calls these givens; clues is another common term.
Givens define the starting puzzle and stay fixed. Your entries fill the remaining cells.
Digital Sudoku interfaces often distinguish givens from player-entered digits visually, but the row, column, and box constraints apply to every solved digit in the grid.
A player entry that creates a duplicate does not change the puzzle's rules. It means the current player state contains an error.
What counts as a valid placement?
Before placing a digit in an empty cell, ask three questions:
- Is that digit already in the row?
- Is that digit already in the column?
- Is that digit already in the box?
If the answer to any question is yes, the digit is not legal in that cell.
If the answer to all three is no, the digit is a legal candidate — but legality alone does not prove that it is the solution for the cell. Several digits may be legal at the same time.
That distinction is fundamental:
- legal candidate: the current placed digits do not immediately violate the rules if this digit occupies the cell;
- forced placement: logic proves that this digit must occupy the cell;
- guess: you choose among unresolved possibilities without proving which one is correct.
Sudoku solving is largely the process of turning legal possibilities into proven eliminations and forced placements.
What makes a completed Sudoku valid?
A completed classic Sudoku is valid when every cell is filled and all three constraints hold everywhere in the grid.
You can check validity unit by unit:
- every row contains nine different digits, exactly 1–9;
- every column contains nine different digits, exactly 1–9;
- every 3×3 box contains nine different digits, exactly 1–9.
A full grid with a duplicate in any row, column, or box is not a valid Sudoku solution, even if all 81 cells contain digits.
A particular published puzzle is also normally expected to have one solution from its starting clues. That is a property of the puzzle's clue set rather than a fourth structural rule for completed grids.
A worked example: Is this placement legal?
What is not a rule of classic Sudoku?
Many common questions come from mixing classic Sudoku with solving conventions or with rules from other puzzle variants.
Diagonals do not matter unless the variant says they do
The two long diagonals of a standard Sudoku do not have to contain 1–9 uniquely.
Some variants, often called Diagonal Sudoku or Sudoku X, add one or both diagonals as extra units. Unless the puzzle explicitly gives that additional constraint, only rows, columns, and 3×3 boxes matter.
The digits do not have to add up to anything
Classic Sudoku has no sum rule.
The digits are labels from 1 through 9. Their numerical values do not create an arithmetic condition. Variants such as Killer Sudoku may introduce cages and sums, but those rules are not part of classic Sudoku.
A digit can repeat across the whole grid
Yes. Every digit appears multiple times in a completed Sudoku.
The restriction is local: the same digit cannot repeat within a single row, column, or box. In a completed classic 9×9 grid, each digit appears nine times overall — once in every row and once in every column.
Pencil marks are not part of the rules
Pencil marks are a solving aid. They record candidates inside unsolved cells and can make difficult relationships easier to see.
You may solve an easy Sudoku without writing candidates at all, or use full candidate notation on a harder puzzle. Neither choice changes the underlying rules.
Solving techniques are deductions, not extra rules
A Naked Single, Locked Candidate, X-Wing, or Chain does not introduce a new constraint to the puzzle.
Each technique is a repeatable logical argument showing that a candidate must be placed or eliminated because of the original Sudoku constraints.
Does every Sudoku have one solution?
A properly constructed classic Sudoku puzzle is normally expected to have a unique solution.
However, the statement needs precision. The row, column, and box rules describe valid completed grids; they do not guarantee that every arbitrary set of starting clues has exactly one completion.
A poor or incomplete clue set might allow multiple solutions. An inconsistent set of clues might allow none. Puzzle constructors and generators therefore validate the clue set separately to make sure the published puzzle has exactly one solution.
Uniqueness becomes relevant to some advanced solving techniques, but it is not necessary for understanding the three basic rules.
How Sudoku variants change the rules
Sudoku variants usually keep some or all of the classic structure and add another source of restrictions.
Examples can include diagonal units, irregular regions, arithmetic cages, thermometer paths, dots between neighboring cells, or rules that forbid certain digit relationships.
The correct rule set is always the one stated for that puzzle. When no extra rules are given, a VeyraPlay guide referring to classic Sudoku means the standard row, column, and 3×3 box constraints.
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 contain two placed copies of the same digit in one row. If that happens, at least one player entry is wrong.
Does every 3×3 box need every digit from 1 to 9?
Yes. Each 3×3 box contains the digits 1–9 exactly once in a completed classic Sudoku.
Can the same number appear twice in a 3×3 box if it is in different rows?
No. The box constraint applies independently of the row and column constraints. A digit can appear only once inside each 3×3 box.
Is guessing against the rules?
Guessing does not create a new structural rule violation if the guessed digit is still legal, but it is different from logical solving. A logical solution justifies each placement or elimination from the grid instead of choosing an unresolved possibility at random.
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 constraints to make your first deductions. Sudoku Candidates explains the difference between digits that are merely legal and digits that have been logically forced.