How to Play
- Fill the 9×9 grid so each row, column, and 3×3 box contains digits 1-9
- Click a cell, then click a number to place it
- Use Notes mode to add pencil marks for candidates
- Use Undo to reverse mistakes
- Get Hints when you're stuck
- Track your mistakes — try to complete with zero!
Sudoku Techniques
Scanning
Look at each row, column, and 3×3 box to find cells where only one number can fit. This is the most basic technique and solves many cells.
Cross-Hatching
For each number (1-9), scan rows and columns to eliminate cells in a 3×3 box where that number cannot go. Often reveals hidden placements.
Naked Singles
When a cell has only one possible candidate (using notes), that must be the correct number. Use the Write Notes button to see all candidates at once.
Hidden Singles
When a number can only go in one cell within a row, column, or box — even though that cell has other candidates — it must be that number.
Naked Pairs
When two cells in a row, column, or box share the same two candidates, those candidates can be eliminated from all other cells in that unit.
X-Wing
When a candidate appears in exactly two cells in two different rows (or columns) and forms a rectangle, eliminate that candidate from other cells in those columns.
Swordfish
An extension of X-Wing across three rows and three columns. When a candidate appears in exactly two or three cells in three rows, eliminate it from other cells.
XY-Wing
Three cells with candidates XY, YZ, XZ form a chain. The cell that sees both endpoints can have Z eliminated. A powerful technique for expert puzzles.