Expert Sudoku

What expert means here

We grade puzzles by solving them with human techniques. An expert puzzle is one where singles, locked candidates and naked pairs run out before the grid is full. To go further you will need techniques such as hidden pairs, X-wings or XY-wings, or a careful chain of reasoning about two candidates.

Expert puzzles start with around 22 to 28 digits. The number of givens matters less than which techniques the puzzle demands.

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.
• 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.
• 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.

These are described with examples on our techniques page.