Understand the three units
A Sudoku grid has 81 cells arranged in nine rows and nine columns. Thick lines divide the grid into nine 3×3 boxes. A cell therefore belongs to three units at the same time: one row, one column, and one box. Any number you place must satisfy all three units, not just the area you happen to be scanning.
The starting digits are called givens. They cannot be moved or replaced. A well-formed puzzle has one complete solution, and every empty cell can ultimately be resolved from the givens and the deductions they support. Sudoku0 publishes only grids that have been checked for exactly one solution.
What makes a move legal
Before entering a digit, look across the cell’s row, down its column, and through its box. If the digit already appears in any of those units, it cannot go in that cell. The digits that survive this elimination are the cell’s candidates. A legal candidate is not automatically the correct answer; it is simply not ruled out yet.
A completed grid is valid only when every row, column, and box contains all nine digits without repetition. Filling every cell is not enough. An incorrect early entry can still produce a full-looking grid that violates a unit, which is why careful candidate checking matters.
A reliable first scan
Begin with rows, columns, or boxes that already contain many givens. Look for a missing digit and test the remaining empty cells. Then switch perspective: if a row feels stuck, inspect the intersecting boxes; if a box feels stuck, inspect the rows and columns running through it. Alternating views often exposes information that one-direction scanning misses.
- Choose a nearly complete row, column, or box.
- List the digits that are missing from that unit.
- Rule each missing digit out of cells where a peer already contains it.
- Place a digit only when one cell or one candidate is forced.
- Repeat the scan after every placement because the candidate landscape has changed.
Why guessing is unnecessary
Guessing hides the reason a puzzle works and can create a long chain of consequences that is hard to unwind. When no move is obvious, add candidate notes, rescan for singles, or move to a different unit. More advanced puzzles require patterns, but those patterns are still deductions. The goal is not merely to finish; it is to make each move for a reason you can explain.
Frequently asked questions
Can a digit repeat diagonally?
Yes. Classic Sudoku places no restriction on the two main diagonals unless the puzzle is explicitly labelled as a diagonal variant.
Do I need mathematics to solve Sudoku?
No. The digits function as nine distinct symbols. Sudoku depends on elimination, pattern recognition, and logical placement rather than calculation.
Can a valid Sudoku have more than one answer?
A properly published classic puzzle should have exactly one solution. A grid with multiple solutions is underconstrained and should not be treated as a certified puzzle.