O‘zРусEng

Hard sudoku: Hard, Expert, Master

On many sites “hard” simply means few digits. Here the level comes from the solving path: a program solves the puzzle like a person and records which techniques were needed. Hard really is hard, but it never requires a guess.

Open Hard levelExpert levelMaster level

Free · no sign-up · phone and desktop

A hard puzzle filled with notes

How the three levels differ

LevelHardest stepWhat to expect
Hardnaked and hidden triples, quads, or four or more pairs and locksimpossible without notes; 15 to 40 minutes
ExpertX-Wing, Swordfish, XY-Wing, XYZ-Wingwhen singles run out you hunt for patterns; 20 to 60 minutes
Mastercoloring, X- and XY-chains, or three or more fish and wingsseveral long deductions; up to an hour

How to approach a hard puzzle

  1. First take all the singles: check each digit from 1 to 9 box by box.
  2. When singles run out, add the notes with “Fill”. From here every deduction trims the notes.
  3. Look for locked candidates: where a digit in a box sits in just one row or column.
  4. Then pairs and triples: two cells with the same two candidates are the quickest to spot.
  5. Still stuck? Pick one digit and look at all of its notes: that is how X-Wings and chains are found.
A selected digit lights up across the board, in notes too: tap its key first to follow a single digit.

A hint when you are stuck

The hint picks the easiest step available: if no X-Wing is needed, it will not mention one. First it names the technique and the area, so you can try to find it yourself. The next stage shows the cells, candidates and lines. Only then does it apply. That way every hard puzzle teaches you a new technique. All the techniques with examples.

FAQ

Can Master really be solved?

Yes, a program has checked that human techniques solve each puzzle to the end with no guessing. You do need to know chains and coloring, and the hints explain those too.

Don’t fewer digits mean harder?

Not always. A 24-clue puzzle can fall to singles alone while a 34-clue one needs a chain. That is why we measure the technique needed rather than the digit count.

What if I make a mistake on Hard?

With “Show mistakes” on, the digit turns red at once. With it off, the hint finds the mistake first, even one in your notes.

See also

Sudoku techniquesSudoku techniquessingles to X-Wing, with examplesDaily sudokuDaily sudokuthree a day, with a leaderboardSudoku duelSudoku duelrace a friendSudoku rulesSudoku rulesfor beginners
Open Hard level