How to play
- Fill every cell with a digit from 1 to 9.
- Every row, column and 3×3 box holds each digit exactly once.
- The shaded digits are givens: they’re locked in.
Controls
| Action | Touch / mouse | Keyboard |
|---|---|---|
| Pick a cell | Tap it | Arrow keys, Home, End |
| Place a digit | Tap it on the pad | 1–9 |
| Pencil marks | Pencil, then digits | N toggles pencil mode |
| Clear a cell | Erase | Backspace, Delete or 0 |
| Undo | Undo button | Ctrl/⌘ + Z |
Placing a digit clears it from the pencil marks in its row, column and box. That’s just tidying up, not a deduction. Fill candidates writes every cell’s possible digits for you, free and as often as you like. Just bookkeeping — the thinking is still yours. It never places a digit.
With Highlight duplicates on (the default), a digit that appears twice in a row, column or box turns red. It only checks the rules mechanically and never tells you whether a digit matches the solution.
Hints (3 per puzzle) first check your board for mistakes, which is free. Then they teach the next step: the technique’s name, why it applies, the cells it rests on outlined, and the digit it proves filled in. Best times only count puzzles solved without hints.
The five techniques
Every puzzle here is checked by a solver that uses only these, easiest first. Easy boards need singles, Medium boards need a pair, Hard boards need a Pointing Pair. Hints and the solver use the same names.
- Naked Single: A cell that sees every digit but one must be that one.
- Hidden Single: A digit with only one possible spot in a row, column or box goes there.
- Naked Pair: Two cells in a house with the same two candidates claim those digits: cross them off everywhere else in the house.
- Hidden Pair: Two digits that only fit in the same two cells of a house own those cells: cross off their other candidates.
- Pointing Pair: When a digit’s spots in a box all sit in one row or column, it can’t go anywhere else in that line.