How to play Sudoku
Sudoku is played on a 9×9 grid divided into nine 3×3 boxes. Some cells are already filled in. Your goal is to fill in every empty cell.
The rules
- Each row must contain the digits 1 to 9 exactly once.
- Each column must contain the digits 1 to 9 exactly once.
- Each 3×3 box must contain the digits 1 to 9 exactly once.
- Every puzzle has exactly one solution, and it can always be found by logic, without guessing.
Playing
- Tap a cell, then a number, to place a digit.
- Turn on Notes to write candidates in a cell.
- Hint shows the next logical step and explains it. Press Apply to use it.
- A wrong digit counts as a mistake. After 3 mistakes the game is over (you can turn this off in Settings).
Keyboard shortcuts
- Arrow keys
- Move between cells
- 1 – 9
- Place or note a digit
- Backspace / Delete
- Erase
- N
- Turn notes on or off
- Ctrl + Z
- Undo
- Ctrl + Shift + Z / Ctrl + Y
- Redo
- Space
- Pause or resume
Solving techniques
These are the techniques the hint system uses, from easiest to hardest. Each difficulty level needs the ones before it.
Naked Single
A cell has only one possible digit left, because every other digit already appears in its row, column or box.
Needed from Easy
Hidden Single
In a row, column or box, a digit fits in only one cell, even if that cell has other candidates.
Needed from Easy
Pointing Pair / Triple
Inside a box, a digit can only go in one row (or column). It can be removed from the rest of that line outside the box.
Needed from Medium
Box/Line Reduction
Inside a row (or column), a digit can only go in one box. It can be removed from the other cells of that box.
Needed from Medium
Naked Pair
Two cells in a unit share the same two candidates. Those two digits must go there, so they can be removed from the rest of the unit.
Needed from Hard
Hidden Pair
Two digits can only go in the same two cells of a unit. Those cells can lose all their other candidates.
Needed from Hard
Naked Triple
Three cells in a unit use only three digits between them. Those digits can be removed from the rest of the unit.
Needed from Hard
Hidden Triple
Three digits can only go in the same three cells of a unit. Those cells can lose all their other candidates.
Needed from Hard
X-Wing
In two rows, a digit fits in only the same two columns. It must sit on two opposite corners, so it can be removed from the rest of those columns.
Needed from Expert
XY-Wing
A pivot cell {X,Y} sees two wings {X,Z} and {Y,Z}. One wing must be Z, so any cell seeing both wings cannot be Z.
Needed from Expert