Hard sudoku
Play sudoku
Hard sudoku, where notes take over
Hard grids are where pencil marks stop being optional. Once the singles run out, you need to see groups of cells that share their candidates: pairs and triples. They rule out candidates elsewhere, until a single appears again.
What it takes
A naked pair is two cells of a row, column or box that can only hold the same two digits; a hidden pair is two digits that can only go in the same two cells. Triples are the same idea with three. Each one clears candidates from the cells around it.
And everything from Easy and Medium.
How long it takes
Expect fifteen to thirty minutes, more at first.
Tips
- Fill in the notes of a whole box or row before looking for pairs: one missing note hides them.
- Look for cells with only two candidates: two of them with the same pair in one row, column or box make a naked pair.
- After every elimination, go back to singles first: they are the quickest wins.
- Tidy notes, a setting that is on by default, takes a placed digit out of the notes around it.
Questions
Why is hard sudoku so much harder than medium?
Because the next step is no longer a digit to place. Pairs and triples remove candidates, and you only see them with notes. Once you know them, hard grids become steady work.
Do hard sudokus need guessing?
Never here. Every puzzle has one solution, reached by logic: the hint button shows the next step, never a guess.
How can I practise pairs and triples?
Practice has puzzles for each of them: find the step in a real position, then move on to the next one.