How to play Sudoku
Sudoku takes a minute to learn and a lifetime to master. This guide covers the rules, the two techniques that solve most easy puzzles, and what to do when you get stuck.
The rules
The grid has 9 rows, 9 columns and 9 boxes of 3×3 cells. Fill every empty cell so that:
- each row contains the numbers 1 to 9 exactly once,
- each column contains the numbers 1 to 9 exactly once,
- each box contains the numbers 1 to 9 exactly once.
That's all. No arithmetic is involved: the numbers are just symbols. A good puzzle has exactly one solution, and you can reach it by logic alone.
Technique 1: the last number that fits (Naked Single)
Pick an empty cell and look at its row, column and box. Cross out every number you see there. If only one number is left, it must go in that cell.
Example: a cell's row already has 1, 2, 5 and 7, its column has 3, 4 and 9, and its box has 6. The only number missing from all of them is 8, so the cell is 8.
Technique 2: the only place for a number (Hidden Single)
Now turn it around. Pick a number, say 4, and a box that doesn't have a 4 yet. Check each empty cell of that box: can a 4 go there, or does its row or column already have one? If only one cell is left, the 4 goes there, even if that cell could hold other numbers too.
This works for rows and columns as well. Scanning one number across the whole grid, box by box, is the fastest way to make progress at the start of a puzzle.
Use notes
On harder puzzles you will reach a point where no cell has an obvious answer. Switch to notes mode and write the possible numbers (the candidates) in small digits in each cell. Patterns that were invisible become easy to spot, and every number you place lets you erase candidates in its row, column and box.
When you get stuck
- Scan one number at a time, from 1 to 9, through every box.
- Look at the rows, columns and boxes that are almost full first.
- Fill in notes, then look for cells with a single candidate left.
- Never guess. If you can't find a move, there is a technique you haven't spotted yet.
In Treptes Sudoku, a hint shows you that next step in stages, and tells you which technique it uses. If you made a mistake earlier, the hint finds it first.
Beyond the basics
Harder puzzles need more advanced techniques: locked candidates, naked and hidden pairs and triples, X-Wing, Swordfish and XY-Wing. You don't need to study them first. Play, use a hint when you are stuck, and you will meet each one when a puzzle needs it.