Expert Sudoku

What expert means here

We grade puzzles by solving them with human techniques, simplest first: naked singles, hidden singles, locked candidates and naked pairs. An expert puzzle is one where all of those run out before the grid is full. Every expert puzzle still has exactly one solution, reachable by logic, but you'll need at least one technique beyond the standard toolkit to reach it.

Expert puzzles start with at least 22 digits. The number of givens matters less than which techniques the puzzle demands: some expert puzzles have more givens than a hard one, but hide one awkward step.

Advanced patterns to look for

• Hidden pair: two digits that can only go in the same two squares of a unit. Those squares can hold nothing else, so clear their other candidates. The same idea extends to triples and quads.
• X-Wing: a digit that can only sit in the same two columns in two different rows. It must take opposite corners of that rectangle, so remove it from the rest of both columns.
• Swordfish: the same idea as the X-Wing, stretched over three rows and three columns.
• XY-Wing: three two-candidate squares linked like a hinge (XY, XZ and YZ). Any square that sees both XZ and YZ cannot be Z.

Each guide works through the pattern on a real grid, with the eliminations shown.

How to approach an expert puzzle

1. Get every easy digit first. Singles, locked candidates and pairs still do most of the work. Expert puzzles usually need one or two advanced steps, not dozens.
2. Write complete notes. Advanced patterns live in the candidates. A missing candidate can create a pattern that isn't really there, and a wrong elimination from a false pattern can break the whole grid.
3. Search one pattern at a time. For fish patterns (X-Wing, Swordfish), take one digit and look only at where it can go in each row, then in each column. For wings, list the squares with exactly two candidates and look for three that link up.
4. Check before you eliminate. Say the pattern out loud: which squares, which digit, why the eliminations follow. It's slower, but a single wrong removal at this level costs far more time than it saves.
5. Go back to singles after every elimination. One advanced step often opens a long run of easy digits.

When you're stuck

Turn on Show mistakes (if you've turned it off) to check for a wrong digit; Undo goes back up to 200 moves. The hint button fills one square. Hints name the technique when it's a single; when no single is left, the hint fills a square directly, and the step you were missing is usually one of the patterns above. See common mistakes and how to get unstuck for a full routine.

For the full list of techniques in the order you'll need them, see the techniques page. Want a daily challenge at this level? The daily puzzle has an expert option, the same for everyone on the same date.