Sudoku Foundations
Everything you need to know before your first technique. This lesson covers the structure of a Sudoku grid, the three core rules, and how to read a puzzle so you can start solving with confidence.
What Is a Sudoku Grid?
A standard Sudoku puzzle is played on a 9×9 grid. That grid is organized into several overlapping structures, each of which plays a role in how you solve the puzzle.
Rows
9 horizontal lines running left to right across the grid.
Columns
9 vertical lines running top to bottom down the grid.
Boxes
9 non-overlapping 3×3 regions outlined by thicker borders.
Cells
81 individual squares (9 rows × 9 columns) where digits are placed.
Givens
The pre-filled numbers printed in the puzzle at the start.
Units
A general term for any row, column, or box — each contains exactly 9 cells.
The Three Rules
Sudoku has only three rules. Every valid puzzle and every solving technique is built on these constraints.
1. Each Row Contains 1–9 Exactly Once
Look at any horizontal row in the grid. When the puzzle is complete, every digit from 1 to 9 will appear in that row exactly one time — no duplicates and no missing numbers.
2. Each Column Contains 1–9 Exactly Once
The same constraint applies vertically. Every column must contain each of the nine digits exactly once when the puzzle is solved.
3. Each 3×3 Box Contains 1–9 Exactly Once
The grid is divided into nine 3×3 boxes (sometimes called blocks or regions). Each box must also contain every digit from 1 to 9 with no repeats.
Key insight: Every cell belongs to exactly one row, one column, and one box. These three overlapping constraints are what make Sudoku solvable through logic alone — no guessing required.
How to Read a Puzzle
When you first look at a Sudoku puzzle, here is what you need to understand:
Empty Cells Need to Be Filled
Your goal is to fill every empty cell with a digit from 1 to 9 so that all three rules are satisfied simultaneously. The puzzle is complete when all 81 cells are filled correctly.
Givens Cannot Be Changed
The numbers already printed in the grid are fixed. They are your starting clues and must remain in place throughout the solve. You only fill in the empty cells.
Every Puzzle Has Exactly One Solution
A well-constructed Sudoku puzzle has one and only one valid solution. This means every cell can be determined through logic. If you find yourself needing to guess, there is a logical step you have not yet spotted.
Your First Move
When you are staring at a puzzle for the first time, here is a simple strategy to get started:
- 1Look for units with few empty cells. Scan every row, column, and box. If a unit already has 8 of its 9 digits filled in, the missing digit is immediately obvious.
- 2Check which digits are missing. For any unit you are examining, mentally list which digits from 1 to 9 are not yet present. The fewer that are missing, the easier the deduction.
- 3Place the digit and repeat. Once you confidently determine a digit, fill it in. Each placement gives you more information for the next move.
Next Steps
Now that you understand the grid and the rules, you are ready to learn your first real solving technique. The next lesson walks you through solving a real puzzle step by step.
Next: Your First Puzzle →