How to Play Sudoku: A Step-by-Step Guide
You play Sudoku by filling a 9x9 grid with numbers 1 through 9 so that each row, each column, and each 3x3 box contains all digits exactly once. The game begins with some numbers already placed as clues, and your goal is to logically deduce the rest. This guide breaks down the process into simple, manageable steps, starting with understanding the game's framework and moving to your first solving techniques. For a broader look at the beginner's journey, see [How to Solve Sudoku for Beginners](/blog/how-to-solve-sudoku-for-beginners).
The Grid and the Rules
The classic Sudoku puzzle is a 9x9 grid, divided into nine smaller 3x3 boxes, often outlined with a thicker border. The core rule is simple but strict: every row, every column, and every 3x3 box must contain all digits from 1 to 9, with no repeats. A puzzle starts with some 'givens'—numbers already filled in to provide a unique solution. Your task is to use pure logic, not guesswork, to find the remaining numbers. Understanding this structure is the absolute foundation of the game.
- Try This: Look at a puzzle. Mentally trace the row, column, and 3x3 box for one of the given numbers to see how the rule applies.
Your First Puzzle: Start by Scanning for Singles
Your first move in any puzzle is a thorough scan. Look for the easiest numbers to place: the singles. A Naked Single occurs when a specific empty cell has only one possible candidate left because the other eight digits are already present in its row, column, or 3x3 box. This is the simplest logic in Sudoku. More commonly, you will find Hidden Singles. This happens when a digit has only one possible cell left in a particular row, column, or box, even though that cell might have other candidates. For example, if a row already has the numbers 1 through 8, the only cell in that row that can hold a 9 is a Hidden Single.
- Try This: Pick a number, say 5. Scan each row and column to see if any are missing only one 5. Then check each 3x3 box the same way.
Using Pencil Marks to Track Possibilities
As puzzles get slightly harder, you can't keep all possibilities in your head. This is where 'pencil marking' becomes essential. In an empty cell, you lightly write small numbers representing all digits that are not yet ruled out by the row, column, and box. For example, if a cell's row contains 1, 2, and 3, and its column contains 4 and 5, and its box contains 6, the initial pencil marks for that cell would be 7, 8, and 9. This visual aid transforms the puzzle. You can now see patterns and narrow down options. Updating these marks as you place new numbers is a key part of the solving process.
- Try This: On a fresh puzzle, fully pencil mark one 3x3 box. Watch how placing one number in that box lets you erase that candidate from other cells in the same row, column, and box.
Your First Technique Beyond Singles
Once you are comfortable with singles and pencil marks, you can use them to find your next logical step. A common beginner technique is to look for a cell where all but one of its candidates are eliminated by other pencil marks in its unit. Suppose a cell has pencil marks 3, 7, and 9. If you check its row and see that another cell must be a 3 and another must be a 7, then the original cell can only be a 9. This is essentially the logic behind finding a Hidden Single using your notes. This process of elimination, supported by accurate pencil marks, is the gateway to more advanced strategies.
- Try This: Find a cell with two pencil marks. Check if one of those digits is forced to be elsewhere in the same row, column, or box. If so, you've solved the cell.
Key Facts
- ▪Sudoku is played on a 9x9 grid subdivided into nine 3x3 boxes.
- ▪The core rule is that every row, column, and 3x3 box must contain all digits from 1 to 9 exactly once.
- ▪A puzzle begins with 'givens'—pre-filled numbers that guarantee one unique solution.
- ▪A Naked Single is a cell that has only one possible candidate number remaining.
- ▪A Hidden Single is a digit that can only go in one specific cell within a row, column, or box.
- ▪Pencil marking involves writing small candidate numbers in empty cells to visualize possibilities.
- ▪Systematic scanning for singles is the recommended first step for any new puzzle.
- ▪The logic is deductive; guessing is not part of proper Sudoku strategy.
- ▪Placing a new number immediately eliminates that candidate from its row, column, and 3x3 box.
Frequently Asked Questions
Can I use the same number twice in a 3x3 box?
No. Each 3x3 box, like each row and column, must contain the digits 1 through 9 with no repetition.
What is the easiest way to start a Sudoku puzzle?
Scan each row, column, and 3x3 box for missing numbers. Look for Naked Singles or Hidden Singles to make your first placements.
Is guessing allowed in Sudoku?
No. Classic Sudoku is solved through logical deduction only. Guessing can lead to multiple contradictions and is not the intended method.
What are pencil marks for?
Pencil marks help you track all possible numbers for an empty cell. They are essential for spotting patterns and applying logic as puzzles get harder.
How do I know if my Sudoku puzzle has a unique solution?
Properly constructed puzzles from reputable sources have one unique solution. The 'givens' are designed to allow only one logical path to completion.