What are the rules of Sudoku?
A standard Sudoku puzzle uses a 9x9 grid. Your job is to fill the empty cells with the numbers 1 through 9 so that three conditions are always true. First, each row must contain every digit from 1 to 9 once. Second, each column must contain every digit from 1 to 9 once. Third, each 3x3 box must also contain every digit from 1 to 9 once. No repeats are allowed inside any row, column, or box.
That is the rule set in its pure form. Nothing in the puzzle depends on arithmetic. The numbers are symbols, not values to add together. What matters is placement and uniqueness.
How does Sudoku work?
A Sudoku board starts with some numbers already filled in. These are usually called clues or givens. They are the fixed anchors of the puzzle. You use those clues to work out what belongs in the empty cells. Every move comes from checking what numbers are missing in a row, column, or box, and then seeing which cells can still accept them.
The puzzle works because each empty cell belongs to three overlapping structures at once: one row, one column, and one box. The answer must satisfy all three. That creates a web of limits. As you place more correct numbers, the web gets tighter and the remaining answers become easier to see.
What is the goal of Sudoku?
The goal is not merely to fill the board. The goal is to complete the board correctly according to the rules. A finished Sudoku must have every row, column, and box filled with the digits 1 through 9 once each. If you complete the grid but a row contains two copies of the same number, the puzzle is not solved.
How many numbers are in Sudoku?
A completed standard Sudoku grid contains 81 cells. Each completed row contains the digits 1 through 9, and since there are 9 rows, the puzzle uses nine copies of each digit in total. The interesting part is that the puzzle does not start full. Some cells are blank, and the number of clues changes from puzzle to puzzle.
Can Sudoku have multiple solutions?
A well-made standard Sudoku should have one unique solution. That is the normal expectation in serious puzzle design. If a board can be completed in more than one valid way, then the player cannot rely fully on logic to prove every move. At that point the puzzle becomes weaker as a logic challenge, because different answer paths can lead to different finished boards.
Why should Sudoku have only one solution?
Uniqueness matters because Sudoku is meant to be a logic puzzle, not a guessing game. If there is only one solution, then every correct move can in theory be justified by logic. You might not always see that logic quickly, but it is there. If a puzzle has two solutions, some positions may remain ambiguous. The player can feel stuck not because they missed a pattern, but because the puzzle itself fails to provide a unique answer.
What happens if Sudoku has two solutions?
If a Sudoku has two valid solutions, it is usually considered flawed as a standard puzzle. A player may still complete it, but the experience is different. The puzzle stops being a clean test of deduction and starts drifting toward uncertainty. For most published Sudoku boards, this is not desirable. Good generators and constructors try to avoid that problem entirely.
What makes a Sudoku puzzle valid?
A valid Sudoku puzzle needs at least three things. First, the starting clues must not already break the rules. Second, the puzzle must be solvable. Third, a standard high-quality puzzle should have one unique solution. Those conditions together make the board correct enough to publish or play seriously.
Difficulty is a separate question. A puzzle can be valid and easy. It can also be valid and hard. Correctness comes first. Challenge comes second.
What makes a Sudoku puzzle correct?
A correct Sudoku puzzle has an internally consistent set of clues and leads to one valid finished grid. The solved grid itself must obey the row, column, and box rules everywhere. A board that looks complete but contains a hidden duplication is not correct. A board with two different valid endings is also not correct in the usual standard sense.
How beginners should use these basics
If you are just starting out, the most useful thing to remember is that Sudoku is a placement puzzle with strict limits. The basics are enough to begin. You do not need advanced strategy terms right away. Open an easy Sudoku board, check which numbers are missing from a row, then test the open cells against the matching columns and boxes. That simple method already uses the full rule set correctly.
As you gain confidence, move into how to play Sudoku for a calmer step-by-step beginner routine or explore difficulty and generation to understand why one puzzle feels simple and another one feels stubborn. Once the rules are solid, the rest of Sudoku becomes much easier to learn.